Another boy thing I guess

I'd really like someone to explain to me why my son thinks the words penis, butt & underwear are so funny.

Even at the park I heard him attempting to tell another boy a joke and the word penis was involved. I personally don't think it's funny. 2 nights ago after repeatedly being warned to choose different words this is what I hear. " Raise your hand if you want to see my penis".

Alyssa says: mama Christopher is saying potty words.

I mean am I making too much of a big deal about this? He knows it's not okay because he doesn't make these types of jokes at school.