Depending on his age, if he is an infant or younger child-check for a rash or any bites. If he is 9-12, he could be experiencing puberty and the testicles dropping. Older that, could be an internal issue. Again, you should take your son in to a physician to be properly treated. Issues like these, it is best to go with caution in mind-you would not want to wait and allow any additional problems to arise or worsen.
depending on his age, could he be constipated? is he of school age? I don't want to scare you but there is a lot of bathroom bullying, molestation going on and I would get him to a doctor now.just to be sure he is safe. you can't be too safe with the little ones. i pray he will be fine, but please take him in to be checked.