My three year old sons keeps complaining that his penis hurts. He has only been potty trained for 4 months, what could this be? I thought it was just because he was erect, but he now knows what that is and calls it "up". What could it be?

Does it hurt when he is not erect? Does it hurt when he urinates?

There is a lot of info left out.

Whether it does or not, I believe this warrants a visit to the peditrician. You need to make sure that he does not have an infection of any kind.