L.B.
I think this is totally normal (I don't care for it, especially in public) but is very normal for boys this age. My son is 4 and sings gross songs he makes up and talks potty talk too, my neice even chimes along and they just crack each other up, they think they are hysterical! My nephew is 4 and he is the same way, so from my experience it is just something they do, but I can't wait until he grows out of it. I really don't know how to get him to stop but I just tell him that we don't talk about pee and poo unless we need to use the restroom. So then he just 'says' he has to pee and poo so he can say pee and poo, so it is never ending. Good Luck!