Ditto Dawn. She wrote exactly what I was going to say.
Your son is now facing the consequences of his choice and behavior.
My mother always told us, "You represent this family. People will judge all of us based on each of our behaviors, good or bad."
It really made me want to make good choices.
You are correct your son was not the only one involved, but that should not concern you. You need to just concentrate on making sure your son understands, this is exactly what happens to people that break the laws in our society. He needs to face the consequences of HIS decisions. This includes honoring other peoples requests of staying away from their children.
Do not harp on him, instead let him move on. Tell him you know this was a one time situation. You understand he is learning a lesson.
My mother always also told us. "I will always love you no matter what. You can do the most horrible thing in the world, but I will be on your side. I may be hurt or disappointed, but I will always be there for you. " This has given me the base to my entire life. I know no matter what happens, SHE will ALWAYS be there. But I would never want to hurt her or disappoint her.
The best thing your son can do? Move on, with his life. He needs to never let this type of thing happen again. He needs to be a model student, get into an amazing college, have good friends that will actually admit their part of any situation. This other boy, should have admitted to his parents he also made this mistake and should have attempted to stand up for his "friend", your son.