do not assume anything,talk with him, find out how what when where. Perhaps it was friends, again assume nothing and walk in with a positive attitude, try to engage and involve him in the decision making by asking his opinion, let him also be a part of the solution. If it comes even partially from him it will be a very compelling situation allowing for resolution with a minimum of hostility.
Good luck, tread very lightly, he is still your son no matter what.