You do not need to rehome the dog. You need to retrain him, and your son needs to know what to do if the situation ever happens again. Trying to insert yourself between two fighting dogs is the fastest way to get bitten.

You, and probably your son as well, should sign up for an obedience class with the dog. It is not so much to teach him tricks as it is to establish good rules of behavior and respect and communication with your dog. Rehoming him would just give the problem to someone else.

He needs to be trained, the sooner the better.