My son's doctor has told me he needs exercise, but I can't force him. His father thinks any contact sport is bad and he shouldn't be playing it at all. I don't want my son to learn to be a quitter. What should I do?
I would suggest that you sit down and brainstorm, maybe with the help of a partner and/or friend, exactly what kind of behaviors you expect of your son if he is to live with you. Your plan should include how much time he may spend on the computer and watching TV, time for homework, time for activities and family time. It would be great if you and his dad had the same rules, but I sense that's not possible.
Next, you should sit down with him and firmly, but calmly, tell him that if he chooses to live with you, there have to be some changes in his behavior at your home. Then, negotiate with him about which physical activities -- other than football -- (walking, biking, roller-blading, golf); the time set aside for homework; and which TV shows he may watch and when.
Just refocus him on, "We are not talking about what you do at your Dad's. That is between you and him. We are deciding what you have to do if you choose to live with me."
Above all, for your son's sake and your own, do not let him go back and forth from his dad's to your house just because he doesn't want to take responsibility. He may choose not to live with you, and although that would be hard for you, he needs to learn that he has choices and sometimes the consequences of choices are difficult for everyone.
Keep your goal in mind that you want your son to be happy and healthy.