I don't have any answeres. My sister has a son that is adhd and lived with his dad for a while. His dad was nuts! and that is putting it lightly, believe me so we couldn't expect much more from that child as the same genes are a part of his dna also. I understand wanting to help him. It sounds like the boy isn't happy with his father at all in order to run away. I understand your concern for him, however, you also have responsiblity for the other children also. If you have the money to go to psychiatrists... not psychologists, that will probably help him to work out some of his issues. If it were mine, I would be so torn. I really feel sorry for you. And I will say a prayer for you everynight. Maybe you can sit him down and say if any drugs are found and anything bad or abnormal goes on, you will have to go back or go to a "military type school" for trouble teens. I don't know if a speech like that will work or not but if he respects you and you sit him down and be very sincere and let him know that the other children will look up to him and he has to make a good impression and you depend on him to do good and be good, and if he doesn't you will have no other recourse but to send him back to his dad's, then maybe, just maybe with a little luck and lots of prayers.... he could change and be alright.
I have heard of kids so unhappy that they act out like that and then when they are where they want to be it is amazing how wonderful they are. And then I have heard of the opposite. But as a mother I know.... believe me I KNOW... that you want to do what you can for your children and it kills you to see them unhappy and hurt.
Like I said, I don't have any answers but I will say a prayer for you.