Well, how old is he? If you are talking about American football, you should defiately speak to his doctor. But this isn't American football season--- if he is talking flag football, that isn't contact, so he could probably do that.
But I'd think this could be cleared up quickly by one phone call to the doctor's office that put his cast on.
good luck!
We also have a orthopedics and sports injuries forum on MH they may have an answer for you ..
Personally I would leave it longer soccer is an impact sport , it may be aswell to ask the doctor his advice on time line ..good luck