Sounds like some chafing, vaseline may help him he can put it on himself if embarrassed.Or as Annie said the doctor may be good
If you really think something is seriously wrong, take him to the doctor (preferably a male doctor) and let him show the doctor the problem. It is not necessary for you to see first, or to see at all. If you really think he is in pain, get him to a medical person who can help him. He is apparently willing to let embarrassment keep him from getting help, but you should still make it possible for him to get help.