You and your DH both need to come to terms with the fact that D has serious mental illness. Schizophrenia and ODD, and substance abuse, each alone, are bad enough to potentially tear apart even intact families. Maybe you guys could go to counseling to figure out how to handle him going forward. Potentially, maybe social services could get involved to help with job training and placement, if D is even capable of holding a job, or possibly government disability and Medicaid and housing assistance if he will never be able to support himself. Also, you will absolutely need to protect yourself both mentally and physically from him, as he has already shown a propensity toward violence to women (his mother.) You aren't wrong for not wanting to live with or be around someone with these behaviors. Your SO will have a hard time coming to terms with D's problems but he will have to if he wants to both help him and save your relationship. 

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I'd get him a nice tent and a sleeping bag, and wish him well.

Yes, I know this sounds harsh and cruel, but he's an adult and you should not have to live with this level of disrespect. It should be a condition of him staying with you that he has a job and is taking his meds/seeing a therapist/psychiatrist. If he's unwilling, then set him free to live his life.

It will likely take him getting arrested again or having a psychiatric crisis before he gets real, sustained help.

But you must promise all of us if he EVER threatens you, you will call the police. You must have documented records of threats or acts of violence.

schizophrenia and possible Opposition Defiant Dissorde.  Means he will remember something that he will believe that happened to him. Example, he dreams we won $5000 in a scratch off lottery.  Two week later he believes he actually won the money but now can't find it. [schizopherebia].    [ODD]. someone stole the money and he's going to fight that person. 
'These people know the medicine stops sexual feelings,  they can't be with there GF.   

They stop the meds to feel rights, but start other troubles,  some reason he didn't want to shovel snow.  You have to try other approaches,  do you a favor,  pay him for it.  Promise to make his favorite meal Sunday.  Throw him out.  You can't have your life revolving around a mentally ill person.  My SD was schizo and bipolar  I could not deal with her.  I had to get up at 5 AM to go to work.  I had to sleep.  I couldn't have a circus going on at 1 am.  Someone had to pay the bills and keep the major medical insurance in place. I could not let this kid drag me to her craziness .  

Im glad you are here and sorry you are here.

Dont feel guilty that you dont like to be treated like cr@p in your own home. And dont feel like you must like this person, especially since he sounds unlikable. Being sick and dealing with your SO' surgery, its a lot to deal with, and then theres him.

I have a mentally ill SD25 Feral Forger, and I doubt that part of her illness is being selfish and unlikable and sucking all the air out of the room. Thats just her core personality. Luckily I dont have contact with her, but the entire time she and I have always just not gotten along, due to her core personality and lack of respect for me and her father and her sister (who doesnt like her either)

He needs to be on his meds if he is to live with you and SO.

Adolescent onset schizophrenia is a serious mental disorder; adolescents with schizophrenia-spectrum illnesses are 4-8 times more likely to perpetrate a violent act than general population groups. Since your SS also shows signs of ODD and has engaged in threatening conduct in the past, action must be taken to control his symptoms and to keep yourself safe.

It is mandatory that your SS continue to take his medication and to undergo consistent treatment from a medical professional. 

I suggest that you do some reading on the subject of adolescent onset schizophrenia as the more you learn about this disease, the better you will be able to both understand your SSs behaviour and help your DH to make decisions on your best course of action.

There is no cure for schizophrenia. The best that can be hoped for is managing its symptoms to allow the sufferer to live the best life possible.

He is Dx'd and he is an adult. Immediately evict him from your home and call the authorities to deal with him.  Once he is made a ward of the State he will no longer have autonomy on his treatment or his life.  He maintains or he goes away until he is reset to maintain.

One of my childhood BFFs, my oldest friend, was Dx'd with Schizophrenia in his early 20s.  His parents busted their butts to get him treatment, support him, etc... Eventually it was beyond their ability to protect him from himself and society from him. So, the worked with the authorities to have him made a Ward of the State.  He is in his early mid 60s, lives in a garage apartment at his parents home, but when he gets out of sorts, #911 and off he goes in the straight jacket in the back of the padded van to be institutionalized until his meds are balanced and he is aligned enough to return home. He is generally not volatile, he does get manic upon occasion, he generally does well when he is alone. Too much time around others or in large crowds in public and he struggles to discern the voices in his head from the voices of others.

The last time I spent time with him was about 5yrs ago when I visited he and his dad. We went to dinner at their go to local small town cafe'.  About an hour into to dinner, he got up, went outside, and isolated himself in the parking lot until his dad and I finished our meal.  He has had a decent life.  He cleans his parent's pool, does their yard, cares for his pet birds.  They live in the country to facilitate his quality of life where things are quiet and calm.

Even with all of that, he does get hauled off for re-balancing periodically when he has a challenging period.

You and your DH need to stop sacrificing your own lives, your marriage, your home, your family, and the younger Skids to SS-18's choices and illness.  Though he is ill, he is also making choices that he must be held accountable for.

Buh-bye spawn, enjoy the life of a Ward of the State which will protect you, make sure you are treated, and protect society from you. Or, grab your tent, head to the woods, and see how your fantasy of tent living during the winters as a Mountain Man. Hopefully the State can medicate your SS-18 effectively and tune out his toxicity. Even if that happens, be wary and segregate him from your life and family as much as possible. I would.

To facilitate the process, web cams. Everywhere. Cover every square inch of your property and document him to the Nth degree all of the time.  Any manic crap, off he goes.  Yes, it is more difficult to deal with a syndromed adult, however, there are ways to take much of their life choices out of their own hands.

Ward of the State. It works.  My young childhood BFF of more than 50 years is proof as he is alive and approaching his mid 60s where most men Dx'd with schizophrenia don't make it past their early 50s. He is part of his family, he is part of his community. Taking his autonomy has given him a far better and longer life that he would have otherwise have had.

IMHO you and daddy have to put yourselves and the younger SKids first and mitigate the risk that SS-18 s.

Take care of you.