I too would at least do a police report of the incident, including the boys actions that started the whole thing. Do that before you take another step. That way should you decide to approach the elderly couple about what the grown son did, you at least have a record of the entire situation in case the elders are not receptive to the second approach.
As for the grown man, I definitely would not try to approach him personally, because the experience has shown you that the individual is not the level headed "lets talk about this" kind. Why put yourself in a position where someone can really get hurt.
If you are not going to press charges, then pick a safe time to talk to the elders about the actions of the grown son, and leave it at that. Just know that "YOU" have done the right thing. In this world of love and hate, we have to be very careful these days and alway keep in mind that we cannot control what other people do.
Thank God that your family is safe at this point, and find the bright side of the situation. Your son did not get hurt, and the lesson to him is not to do that again.
P.P.S, Tell your husband!
Cynthia