What Causes Pain In Penis Of A Child?

My 3-1/2 year old great nephew is always grabbing at his penis. I ask if he has to potty and he says no - my penis hurts. Yesterday I asked and he said it always hurts. I ve glanced at his private area when he s on the potty and nothing appears to be wrong. I know he has detested wearing underwear - including when he was in diapers - from day one. He goes without or discards his underwear whenever he gets a chance and that s perfectly acceptable but I m curious. Dad and mom says he hates his underwear and has always complained about his penis being sore but they too see nothing out of the ordinary. In his early exams, doctors have discovered nothing and given him a clean bill of health. I m 62 years old and have never had children but I babysit him and his sibblings often. Since this concerns me I have to ask if it is a problem male children typically have? The only thing I m aware of is that quite often his bowel movements require a lot of straining but since he has a very healthy diet which includes a lot of apple juice I m guessing that s something that will improve as his system matures.