DS inappropriate behaviour

[OhWhatNoooow]

My 11yo DS has always made references to body parts and jokes around about it sometimes,, like he'll say of the lady next door, 'she has huge boobies!', or 'she punched me in my willy!' (said while clutching it in dramatic gesture). Today we got a letter from the school saying that he was caught looking over the toilet cubicle when another boy was in it.
Now Im thinking that maybe he has a serious problem sexually or psychologically or something. Is this normal or inappropriate behaviour?

just to clarify, the lady next door does not punch him there, his sister does, well she tries. OMG am I raising messed up kids??

At 11 he needs to know that whilst not abnormal (it's a boy thing that persists all their lives I find....) it certainly is no longer acceptable. It looks like the school are onto it, so hopefully it will stop.

Not sexually abnormal but socially extremely immature is what it sounds like to me. How is he coping generally with his friendships and in social situations?