Our daughter is in a cast now. Our secret weapon is Glad Cling wrap for showers...recommended by her orthopedic surgeon! It works great! If it will be a while before he gets it off, you might also consider Dry Pro cast protector. We purchased this for our daughter (kind of pricey, $40) but so worth it as I hear her now splashing and swimming with her friends in our pool!

You just give them the dimensions of the cast, and it will be at your door within 48 hours.