Hygiene can be a sensitive subject but I think it's best to tackle this dilemma head on. You could do a mini lesson in health on bodily hygiene and have the students journal about ways they take care of their bodies. Drawing pictures or writing could show you insight on how he takes care of himself; then you could write back or have mini confrence with the child to discuss other ways to extend his normal routine. Talking to his parents is also an option. Asking them if the son has expressed any troubles with his classmates to then lead you in to a discussion would be a safe way to approach the problem. As long as you approach the situation carefully and show you're expressing the best interest for everyone I believe the problem can be addressed without hurting anyone's feelings.