I am sorry about your son's accident. The first thing you need to do is save the cup. You should also find the packaging and the instruction manual. This is important because we would need to determine through an expert whether there is a design defect or whether the product lacks proper instructions or warnings which led to your son's injuries.
If the cup is defective, then your son would have a products liability case against the manufacturer of the cup. There may also be other parties responsible, such as the seller or even the coaches at the school.
I would need more information from you regarding your son's accident to properly assess your son's sports injury case. Products liability cases can be very complex, and it is important that you speak to an experienced sports injury lawyer in PA or NJ who has handled products liability cases.