You've bumped into your child's friend at several school functions, and he seemed polite enough. That is the most direct approach. But voicing such concerns can be uncomfortable for parents who want to know but don't want to offend.
Nothing is worse than worrying.
As children get older, the issues change but don't necessarily get easier. The party was only one day away. Try an information exchange When young children are involved, it's perfectly reasonable to suggest getting together with the other parents before the kids play together, Melton and other parents say. You may not want to hit them with a safety and morality checklist, but you get a chance to see those parents interact with their children, and to have a conversation. Often the best approach is to offer information about yourself in hopes that they'll reveal the same.