The only thing he really cares about is playing football -- it is the only outside activity that he does. I wanted to make him quit the team but I didn't know if that would just lead to more idle time to do stupid things.
You state that your boy has never exhibited this kind of misbehavior before. I believe that his very public shame and guilt over stealing these cards from fellow students is the most potent and meaningful consequence to his stealing. He is no doubt feeling shame and embarrassment regarding his theft. Continuing to punish him more and more is overkill, in my opinion. Taking more and more privileges away from him will not make him a more honest lad or make him any more sorry for his misdeed. It will only serve to make him more unduly sad and resentful.
We all do "stupid things" in our lives. I hope that you can talk to him and show him that you understand what caused him to act in this dishonest manner. Forgive him. Ask him to tell you how he's going to stop himself from acting in this way again and offer to help him be the honest boy you know him to be. It's time to start offering him encouraging words for who he is and to stop finding more ways to hurt his feelings.