I am sure he wants to stay here. And he can. But his mother can't, and she is a criminal, and the only reason she is here (and he is a citizen) is because of her criminal conduct.
So, the lad can choose. Stay here, or go with mommy. I don't see the moral issue. No one is separating mother and son. They are making that choice.
Personally, I think we should change the law so that only children of legal immigrants are citizens if they are born here.