1.  Plenty of kids are raised in two homes where there are different rules.. and they adapt just fine.

2.  It won't happen at all if you aren't consistent.  your son is old enough to understand your rules and standards of conduct..letting him get away without following rules is teaching him that you don't care about him enough to set expectations.

SS behaves well and follows the rules at ...school/grandma's/scoocer practice.....SS isn't rude and bratty to....friend's parents/teachers....

All kids learn how to act differently in different environments. The might know that they can insult adults in your home but they know that if they went around doing that at school that they would get into serious trouble and so they avoid it at school. 

Your husband is unlikely to change though. Sorry. He has had 13 years of opportunities and has yet to take them. If you want change in  your home, I would either disengage or start implementing my own consequences.

The trouble is, you can't force him to start parenting better, all you can do is change your own behavior. So if you do anything for SS to help DH, I would stop that. Disengage from SS and make sure that DH is the one parenting him, all the time, for everything.

Also make very clear to him (if this is true) that SS's behavior is jeopardizing your marriage.  You already know this kid is not going to mature normally and be a problem for many years to come.

DH isn't going to change unless the pain of not changing is increased.

1. Kids understand perfectly well that they have to behave in school or other activities, why not in two homes? This excuse is just blaming BM because it's easier than actually doing the hard work of parenting.

2. Yeah it takes time! You have to be consistent. If you make a change one day then don't keep it up the next you make zero progress and it's even harder when you try the next time because the kid knows you're weak and won't follow through. What happens then is the kid just holds out because they know you will give up before they do.

Tell your husband that his stance is going to lead to SS not having any respect for his father as a parent and as a man. Being wishy washy and giving in makes him looks weak. Show your son what it means to have conviction and follow through for your values!

To hubby was as follows:

 #1 (our variation was dh blaming bio mum and the divorce as messing up skid(s) personality and behaviour and caused mental issues and why they behave disrespectfully.

my response was “plenty of people divorce, statistically now its more like 2/3 of marriages end in divorce, that doesn’t give you a free pass to behave like a disrespectful ahole and treat your half siblings and stepmum like dogshit!!”

#2 (dh variation was ss needs time to change, he just needs time and i’m confident he will change even 5 yrs into marriage that was the excuse)

my response was at 2.5-3 yrs marriage to tell hubby since he clearly didn’t respect me as his wife, allowed ss20 to abuse me and my 2 young kids with hubby and enjoyed it, hubby enabled it, enabled his adult son to abuse us and encouraged us and still had the same shitty excuse “he needs time to be a decent respectful human” like it was such a ridiculously hard demand, that we need to divorce since hubby was not serious about making active changes. 

Hubby didn’t want a divorce and told me he wouldn’t divorce me and i told him he didn’t have a choice, see he let this shit drag on for years. Its been 3 yrs too long for any stepmum or young kids to have to tolerate this bullshit!! I asked my husband what kind of husband or man actively encourages and enables his son to abuse his wife and 2 young kids.

I told hubby at about 4 years plus of marriage that he had no right to firce me and our 2 kids to live with ss when he is actively causing disharmony in our home... hubby doesn’t wanna be the bad guy and kick him out. Its always ss will make a bullshit apology except he never does and its purely what he says to shut daddy up.. we bought a new home in my country as my kids are schooling there so pre covid me and hubby were flying back and forth.

my husband told ss late last year that he has caused so many problems that no one wants to be around him and for religious holidays hubby will be spending it with us every year and he can’t possibly bring ss along when our whole marriage ss has referred to me as a stranger as justification why he should not have to acknowledge my presence at home or my kids. 

He has told his dad that he is incapable of showing any affection to our kids (his half siblings) so don’t force it as a response to hubby telling him off to stop pretending our kids are invisible. He only shows 1 second of attention and awkwardly touches them when my inlaws are around but his cousins grill him how ridiculous he is to not even treat our kids as his siblings. But of you see him with his 2 sisters (full sisters) and its even more awkward, so I don’t expect much....

i have disengaged long ago from any of my skids and refuse to participate in any visits, meetups or events that they will be at, especially sd’s (they are the worst and such pathetic midwives its so gross, at one point i told hubby from now on he should have sex with them since they clearly were his wives and a priority over and above me everytime), that scared and grossed hubby out because he turned a blind eye to alot of it. They had no respect of boundaries, would undress my daughter, tell me off how to parent my kids... ummm no, not happening ever!!

i told hubby to go to hell if he ever thought i needed to explain myself to his kids or justify my parenting style of my own kids... There is so much drama with the skids... staying away saves my sanity