Your son certainly has a lot to deal with. I was about the same weight at that age and it was difficult enough without the added problems of all that your son has on top of it.
I don't have any experience with ADHD, ODD, etc....but what about joining a gym as a family. Make it fun to exercise by creating your own challenges. Not how fast are you type challenges but more how many minutes have you spent exercising. Very nonconfontrational....just when you reach a certain amount of time exercised, you get a prize.
I wish your son the best and hope you can find a solution that helps.
When one door of happiness closes, another opens:
but often we look so long at the closed door that
we do not see the one which has been opened for us.