He's a permanent resident nothing you can do about that. He may be a minor but that green card is his and his alone. You cannot take it away or stop it. It was done the minute he entered the US. Keeping his not yet CG away form him would also be illegal. The best thing for you to do immigration wise is get him US Citizenship by keeping in the US with his mother. Once she naturalizes he will be a USC. Right now if you try to get him on that plane and he doesn't get on it. Starts a scene or runs away you will be in a lot more trouble, more then you can even begin to imagine.
I know you don't want parenting advice but honestly that is exactly what you need. You said you don't want the younger kids to pick up his bad habits. I truly do understand about bad influences on the younger kids, so I do get your worry. He needs time and talking, he is going through a huge adjustment. In the 5 yrs his mother was away how much time did she go back to spend with him? It really does sound like the two are practically strangers. It's barely 4 months since he arrived.