SS may be too far gone already. Personality is pretty much set by age 10 i think. Maybe earlier. Sometimes a very structured environment such as the military can straighten people out, but can your DH provide that? 

In three years he's going to be a adult.  What do you think DH is going to do. Have fights every other hour. You want to live in a boxing arena? Or a home ?   
SS is already lost   There nothing you or DH can do. Just let it go. SS will not live with you 

He needs to take control now, at least while SS is in your home on visitation. Shut off the Internet, take his phone. He can sit his ass in the dark and brood but he should have zero connectivity in your home. What is he going to do? Fight daddy until the internet is turned back on?  Nothing a back hand knocking his ass out in self defense won't fix.

Make sure you have web cams everywhere so when SS attacks dad and dad knocks his ass out and zip ties him into a quivering mass in the middle of the living room until the police haul his violent ass off that the footage is all ready for court.

Just thinking of the hell that would have unfolded if I had attacked my USMC father makes me shudder even today.  4+ decades later.

I simply do not understand parents that would not rain hell down on a POS kid like this.  There is a reason why this shit was an anomaly back in the day when daddy would take Jr. out behind the woodshed for a talk with the barbers daughter when they stepped out of line.  Now these POS failed family spawn run rough shod over entire families with seemingly no consequences.

Daddy needs to set Jr. straight.  'Pull that shit again in my home and your ass will rue the day you were born. I already rue that day."

Lather, rinse, repeat.

The time for your DH to act was before this behavior pattern became set in stone.. which at 15?  it is pretty far down that road.

Now, I'm not sure why you say that DH couldn't give his child consequences on his own custody time.  He couldn't force BM to do it.. but he could have done it himself.  He could have also tried for more custody claiming BM wasn't being effective with her discipline.

It's not just "punishment".. but teaching, explaining, and providing opportunities for the kids to make good or better choices.  But, yes.. giving consequences for behaviors and actions.. or lack therof is part of the overall process of raising a child.

I don't think your DH will have the time to fix the cluster f his son has become in a year or two.. which is probably best case the amount of time he will get to do it by the time things worm through court.  his son will likely push back anyway.. why would he want to leave his marshmallow of a mother?

It will be a waste of resources for your DH.

Unfortunately, he has likely given them so much rope to burn.. there is nothing left to get a grip on.  he should have stepped up years ago.

Sounds like sd17... definitely a disaster 

Does your DH really think he can take a 15 year old who's been left to his own will for years, and then suddenly lay down the law and the kid is going to just go along? I worked in the Alternative School in our area....filled with kids like you describe. I'll tell you what I told the parents, demanding I do something with their out of control kid...."time to do that was years ago. Not when he/she is almost an adult." Your chances are slim to none and your home will become a war zone. So get ready if your DH's ridiculous plan pans out.

That will end in disaster.  BM has brought this on herself. I know there isn't much, if anything, that your husband can do at this point.   He tried and BM interfered.  I would not want him living under my roof. 

Buy a case of soap on a rope for him.  He's a lost cause and will need life to teach him.  DH lost that position when he gave his balls to BM.  

You husband missed the boat on parenting. He can always have instilled boundaries and rules on his visitation time. Now its too late. And this skid will get worse, so hang on its going to be a wild ride. Start investing in nanny cams and locks, and lock boxes. Learn how to spackle over holes in the wall and how to install new doors for when the current ones get kicked in. Disengage from his kid, because your husband will not suddenly be able to start parenting and be successful at it, since no one has been parenting this kid.

Oh and you wont be off the hook at 18 either, because this skid sounds like a failure to launch in the making. He will fail to launch and then you and your husband will get to support this lazy, mean, rude failed family attempt.

Good luck and keep us posted.

Time to bring in the zero coddling professionals.

Suck it up or be destroyed kid. You will not be around to destroy anyone else.  It is better if you can have him adjudicated to a behavioral modification school.  That way it is not up to daddy, mommy, or anyone else in the family whether or not he gets out.

Convince the school authority and the Judge kid.  That is your only way out before you turn 21.

 Does your DH seriously believe that any judge is just going to hand him full custody (which is extremely rare) because he asks for it? Not gonna happen. Even if it did, what does he imagine he's going to do to get his kid back in line at this late date? 

Even if he did, by some miracle, manage to obtain more custody time, what makes him think his son will stick around? By the time court is over, he'll be of an age where no judge will make him go to his father's if he chooses to stay with his mother. 

Why did it matter if BM didn't approve of his disciplining his son? what happens on his time is none of her business and what happens on hers is none of his. If he'd been doing his job as a parent right from the start, maybe his son wouldn't be such a hot mess. By  the way, exactly how did BM manage to interfere in his time? How much custody/visitation time does your DH have right now?

"DH will file for full custody of SS and then lay down hell to SS without BM's Interference."

Does he realize that this could take years to settle? 

"DH feels BM needs to lose total control( pretty much there) of SS before DH can step in or BM will continue to fight DH regarding Discipline. "

Tell me, why did your dh allow her to fight him regarding discipline? If he can't keep his ex out of his business concerning his parenting, I find myself wondering how on earth he 's going to manage to deal with an out of control, violent teen...

What is his plan B should the kid not respond to him laying down hell? 

As everyone else said, at that point, even at this point it will be too late. 

However, your husband's plan to stop fighting with BM over it is probably correct. He isn't going to win. His plan needs to be- BM made the mess, BM can deal with it. NO, SS can't come live with you. NO, you are paying for a boarding school for him. NO, you don't agree to _________ (whatever plan that costs a ton of $ that BM has). Just keep the status quo over the next couple of years and pray that he pulls his head out of his own a$$ and wants to do better in life. Maybe start encouraging the military for him. Structure, training and decent pay are all strong reasons for him to join- he just has to graduate!! 

Just another observation.  Your DH is also estranged from his older daughter per your post from early last year.  He seems to have a habit of easily writing off his kids... taking the path of least resistance.

1.  Teen daughter and his GF clash.. teen has a hissy as teens are prone to do, GF wants her GONE... He decides against counseling with his daughter (as a conduit to come back home) because GF tells him to.  SD tries to reconcile with DH.. he refuses because "GF".  Years later the GF is gone and he regrets that he didn't man up and be a parent.  

2.  Has a son that is not well mannered or behaved.  Kid is a handful, BM doesn't like his brand of parenting.. he just gives up and lets BM handle "whatever" Kid becomes a bigger pain in the tail as he grows older.  DH has some half baked idea that he will just let the boil come to a complete head and then BM will be willing to thrust the infected abscess of a almost grown man his way and that his son will magically toe the line for him now.. after he has likely "abandoned" him like he did his older sister? (you think that might have colored SS's opinion of his dad too?)

In both cases.. he has taken what might be seen as the easy way out.  He stepped back and out of his children's lives at the first sign of difficulty pretty much.  He did it for his GF... he did it because it was easier than staying the course with his son as a younger child... 

I mean, I hate to say it, but your husband hasn't been a great father to either of his kids.  I don't see how he is all of a sudden going to be that great and effective parent that fixes this boy in the final year or so of his "minorhood".  

Just because he is being delusional about "fixing his son", doesn't mean you have to agree to go along with his plan.  I think someone else suggested something like a military academy.  Whatever it is, I would refuse to have him in your home.. ESPECIALLY if you also have other minor children who live there.  You have an obligation to provide them a safe place to live.  Your husband's scheme could jeopardize that in ways you don't want to contemplate.  The fact is that he wasn't a good father to either of his children.  He gave up on his daughter quite easily it seems... and he did the same with his son.  It may be that he wants to fix it with his boy because he regrets what happened with his daughter.. but hey.. he abandoned one kid due to his GF.. why not do the same for you?  At least you won't suffer and your kids won't right?

As PPs have pointed out, your H is spinning a line of pasive bull that makes BM the bad guy while allowing him to feel better and avoid doing the hard work of actually parenting. Yes BM may suck, but he sucks, too.

OP, what are your thoughts on this plan of non-action? Do you grasp the misery and strife you'll be subjected to if you permit a juvenile delinquent or stunted adult to live with you? This site has countless posts of BMs who marginalize the other parent, raise their kids to be monsters, then punt them to daddy when they either become uncontrollable or the c.s. ends. Dad, thrilled that Little Johnny/Suzie "wants" to live with him, welcomes their widdle terrorist with open arms but seldom lays boundaries or expectations.

You need to get ahead of this NOW. You have veto power and need to lay some truth on your H. Kids like his don't wake up on their eighteenth birthday polite, well adjusted, and really to adult. They're entitled, and lack basic life and social skills. They don't do well in school and can't keep jobs. IF they move out, they often boomerang. Don't be passive about this, OP. Start planting the seeds now, or suffer later.

Taking in YSS as a late teen due to BM not wanting him anymore and trying to fix him nearly ended my marriage.  It is what had me doing Google searches that led me here.  And YSS was an angel compared to the burgeoning thug you are describing.  He was not violent and was basically a big slob but with a reasonably good heart for a teenager as least part of the time.  It still nearly drove me demented.  So let me repeat - Don't do it.

Bahahahaha. Your DH is completely delusional. If he couldn't control SS When he was younger, how is he going to control him as an almost adult? What makes him think BM isn't going to continue to let SS make the rules until he ages out?