R.,
I have been told and I believe that there is a reason that God gives us children in the form of cute, cuddly babies...if we had them in the form of teenagers human race would long ago have been extinct!
I have had a teenager under my care since 1990 when my first son turn 13. My youngest will turn 20 in 2011, four kids the first three are boys so almost everything I learned I am now relearning the 4th time around. But there is one thing that is the same with all four of 'em, I can't change the inside, the feelings, the lack of motivation. I can only change the behavior. Hopefully by changing the behavior the inside will eventually come along with the outside. You know, the ol' you can lead a horse to water, but you can't make him drink technique.
Sit down with him and tell him exactly what you expect from him and exactly what the consequences will be if he doesn't follow through with your expectations. Make sure your husband and his father are in on it. If you don't think your son will actually listen to you without a battle beginning, write it out, it might be a good idea to have it in writing anyway so it can referred back to later on. There won't be any, "I forgots". Be realistic about your expectations and your consequences. Is he really an A student? Or maybe a B- student. Is it more important to have his bed made everyday or the dishes washed? Pick your battles. Make the consequence appropriate for the offense and make sure it is something you can follow through on so you're not punishing yourself in the long run. If you want to ground him for three weeks make sure you can follow through for three weeks and not give in after 10 days. If you think you can only hold out for 10 days then only ground him for 10 days to begin with. The actual length of time isn't as important as the fact that you follow through with what you say and be consistant so he knows what to expect from you.
As for going to live with his father if that is what you and HIS father determine what is best, not your son, my suggestion is to make it clear to him that your homes do not have a revolving door. That he is to stay there a predetermined amount of time and then the issue of where he will live will be discussed again. Don't let him think HE is going to be making the decisions in his life. Remember, he is only 15.
Good luck. None of this is gonna be fun whatever you decide to do. One because you'er dealing with a pain in the butt teen age boy and two you're dealing with your X.
J.