Why?

My sweet son is fantastic at going potty by himself. He's finally starting to learn that he does not have to take all his clothes off to go potty, just pull down his pants and underwear. There is one little hurdle we are still trying to get over. He has the need to tell me and show me every time he has an erection. He comes running (well, waddling, his pants are around his ankles) to me shouting, "My peens is big! My peens is big", I tell him it will get small again after he pulls up his underwear and pants. That has been working, until yesterday. He did his show and tell, I told him to pull up his pants, he did, and then just 60 seconds later he pulled his pants down in the kitchen to tell me his peens is still big. WHAT DO I DO?? How do I explain to a 3 year old that walking around with your pants around your ankles is not ok? How do I explain erections?