What age children are we talking about here? This is a difficult topic to discuss with parents that you have no previous relationship with. If there is a school counselor, as opposed to a teacher, I would speak with him/her. Counselors understand what to look for and how to bring things up and keep them confidential. A counselor woul be more likely to protect your son's confidentiality and keep him out of the situation.
More importantly, it is great that you and your son have such an open and honest relationship. It is shocking what goes on with children these days, and what types of situations they get themselves into. Keep talking, meeting friends, and being involved so your son won't fall in with a crowd that pressures him to do something out of character for him.
I wish you the best of luck. Raising children is a difficult challange.