I'm sure you have some very good responses here.
Why would you even consider getting a divorce because of him? Have you considered or thought about it in the back of your mind? If I had a good marriage and or a marriage worthy of keeping I wouldn't even think to let someone/anyone make me divorce. So I wonder if you're not thinking something beyond this or why you'd be so willing to drop your marriage.
Anyway, obviously this is a big problem. And it didn't just start now, this has been building up and so you must realize this when making your decisions about how to handle it. He obviously needs a sense of peace as do you all. Your other children will be deeply affected by his behavior. And that's something to keep in mind. Have you treated him as a son? He does not need to be a bio son to be treated as one. He should have been treated this way from the start.
Your husband is making a very big mistake to fear him in any way, shape or form. It's too bad his mother has left him and does as she pleases. I've seen this before and it never works out to well. He's better off without her yet your husband cannot put his whole family in jepordy because he's afraid his son will go off with his mother. This will not get better without a lot of prayer, a lot of help and a lot of working together for the good of him and the family.
Do not allow one person, especially a child rule the household. Perhaps your husband should begin with a heart to heart with him off and away from everyone else. And you'll khave to start acting like a mother, although it's probably too little too late. I'm sure he'd feel that way. But people can change when desire for it is their motive.
There is much more here than meets the eye. You say there is family time and all, but he doesn't feel included. Also, it is very painful for him about his mother and that's why she should not be allowed in his life. If he wants her in his life tell him to wait till he's adult and they he can deal with as he sees fit. You two need to be bold enough to think beyond the box with this. Get help if you need it.