Wow that boy is gonna need alot of love. What a horrible life he has been brought up in. I think you and your husband should get some guidance before he even gets there. Get some advice on how to handle him when he arrives. Not only does he have his set of existing problems he is about to loose his mother and he is going to be grieving. You don't know how he is going to deal with that.
I can only imagine that if he walked through the door and you and your husband are laying down a heavy law and packing his bag for rehab that you will loose him before you even have a chance to help him. I would think you need to see what state he is in. Not that you let him take over the house but how about showing him some love that he is probably so starving for. That boy has got alot of pain that he needs to work through. He might not even be able to accept love from you both at first.
You have a very tough situation ahead of you and you and your husband are going to need to stick together. Your husband is going to need you more than ever. Walking away from him or that boy I think could only damage the situation more. You don't know what is ahead yet, don't packs your bags yet. Stand by your husband and support him. This is his challenge but you are a part of it too. Don't let it be your burden, stay strong.
This is probably not going to be easy for you, but you have a wonderful opportunity to help this boy turn his life around and it is going to take lot of love and support. Educate yourselves like crazy, this is not going to be easy but I hope for you a rewarding outcome. It is possible!
Best of luck!