Hi... I have been kind of in and out so I hadn't seen your post. I think I read all the responses, but there are a lot, so I might be repeating what some others have said.
I, too, am a stepparent of 2 damaged children (hence my board name). My 2 are older; I met my husband when Onyxx was 8 and Jett 5 (and, no, those aren't their names - I called them difficult child 1 and difficult child 2 for quite a while). However... There were some similar issues with their bio mom. And 5 years ago, physical custody changed, 3 years ago my husband got full legal custody. It was awful.
So now that you know that bit...
Your husband is playing ostrich. Let me see if I can explain. He does see that there are problems; but admitting it, even to himself, means that it is his fault. Now you know it isn't. Bio mom really messed up. BAD. But - here's the logic: Your husband chose to have a relationship/sex/children with this woman. She chose to abuse/neglect the boys, resulting in her losing custody and having problems. Ergo, he "chose" to put the kids in that position. Nah. Ain't true. But that's down deep, emotional. Surface logic - he knows it's not his fault, but there's that little fear... what if...
ODD is a catchall diagnosis. Truthfully, what child doesn't display a little bit of opposition and defiance? But the ODD diagnosis doesn't say what to do!!!
Let me give you an example... Onyxx keeps everything - she's getting better, but still. Borderline hoarder, even to the point of hiding garbage under her mattress. And this drove me nuts until finally I figured it out (with help from this Board) - there's 2 basic reasons. One: She's been emotionally abandoned a lot. husband never did, but bio mom told her he had; and since she never saw him, she only had bio mom's story to believe. Later, when she stuck to her story that bio's boyfriend had abused her, and would not recant, bio abandoned her, too. Having all this stuff means it's there for her, and it can't abandon her. Two: Bio mom would throw away Onyxx's things (clothes, toys, etc.) whenever she was angry or Onyxx did something bio did not like. Bio destroyed all of Onyxx's things that would not fit in her car when she left, then told Onyxx that Daddy had "stolen" them. (Daddy thought someone had burglarized the apartment and called the police before he found the note, so there were all kinds of pictures of the disaster... When Onyxx finally could ask him about her pink teddy bear, he told her the truth, he'd HAD to throw it away... Onyxx finally asked me, at age 11, about the stuff, and not knowing any better I showed her the pictures...)
So... For YOUR son, can't you make it so that he can close his bedroom door? The other two, I understand. And I bet if you check with the apartment complex, and ask for maintenance's help to temporarily remove the door, and explain the kids are terrified of being locked in even accidentally... I bet they'd be reasonable. If they are NOT reasonable, find a couple of colorful door stops, put each boy's name on one, so they can have control over the door being OPEN. Don't move them, even to vacuum, without the boys' agreement. Perhaps if they are not in the bedroom when you close the door, but YOU are, you quickly vacuum then have THEM put the doorstops back in place?
The older one, especially, is not yet comfortable in your home, and it could take quite a while (years). Given what happened, he is also afraid of being abandoned/neglected by you and your husband, because that is ALL he has known from bio. This will take time. In the meantime, he is pushing, by hitting and hurting - trying to see how much it takes to make you and husband abandon him, too. (Did your husband have visitation with them when they were living with bio?)
NOW. That said, children act out a lot of what they've been through. Onyxx screamed a lot at everyone. (So did bio... I've learned to mostly be calm...) Onyxx hits, because that is what she learned... Jett hides, inside himself, because he learned if he did that, he'd be left alone and not hurt. SO. A neuropsychologist evaluation is very much necessary as is therapy. Therapy for Gavin is necessary, too, because this WILL change his life. I know you have no legal rights over your stepsons... been there done that (am there doing that), but if your husband will not protect his own kids from themselves... And, no, it's not easy to get the ostrich to look up. (But when they finally do, watch out & don't mess with them!)
I know you love your husband. But... You must protect. Gavin. Yourself. Your husband, for that matter. Start with the most vulnerable. And if you have to leave - you have to. I didn't... But... It IS a family joke in my home that if I'd known what I was in for, I'd have RUN SCREAMING AS FAST AND AS FAR AS I COULD.