I would recommend you stay out of it - Unless you contact Children Services and follow their recommendations. The parents not responding to your calls may signal that they do not want your help. Difficult situation - good luck!
Disclaimer - I am a practicing attorney in Ohio. The law varies state to state. This is being offered as general legal education. There is no explicit or implicit offer to provide legal representation.
Worse your son might seek a sexual relationship with his friend and that could ruin his life!
My name is Stephen R. Cohen and I have practiced over 38 years and can be reached at 213-819-1171. I practiced mainly in Los Angeles and Orange County, California. I am not seeking clients from existing relationships with other attorneys, and give only limited advise over the phone (the phone is primarily used to set appointments), these services do not create an attorney client relationship