[nakedbabytoes]

The head of the penis will look red or purpleish compaired to his foreskin, so it may be just fine. And kiddos that age often confuse the "hard on" sensation for "hurting" simply because it feels weird and they don't know what to call it.
But that being said, if you're worried something may be wrong, go see somebody. Bodies usually do best left alone but not always.

My son's favorite thing lately since he started PTing is to push his finger all the way into his foreskin. Like you would sticking your finger into a balloon opening.
Doesn't seem to bother him but I scratch my head!