I would suggest two things: first, take an exmanation for your son and make sure he is really OK. This is the most important thing. It is not the way to save money to risk his health unless you are really sure.
Second, find out what happened and how the accident happened. Ask your son, his friend, find out who witnessed the accident. Keep records of everything. If there is a serious problem or you eventially incur a significant amount of money, talk to an attorney.