That's a sensitive/difficult situation. 
 
I know if it was me as a parent - I would like to know. Maybe you could talk to the parent and tell them you don't want to put your son in a difficult situation. You could suggest to the parent to contact the boy's bishop (similar to a pastor) where he is living now and have the Bishop confront and work with the young man. That way you could let the parents know, but also help provide cover for your son. Plus the Bishop is the right person to help work the situation.

Within our church we have bishops within every area, the church building he would be going to will have one. Try contacting him, He could contact your sons room mate, and just try and help him. that's what i would do. He could very likely assume that his bishop contacted him buy a bishops intuition or inspiration.