Parents of boys, please help

So I am the single grandmother to a 5 year old boy. I had a daughter and I just don't know sometimes what I would consider apprappropriate "boy" behavior. He started kindergarten this year and since then he tells almost daily butt jokes. He thinks the grossest things are hilarious. And he has his hands down his pants all the time. We're sitting here on the couch and I'm looking up firetruck videos for him, he stuck his hands down his pants, farted on them and then said "my hands smell like butt" and tried to get me to smell them. 🤢 I asked him to please go wash him hands and he did. We discuss keeping his hands out of his pants etc... But, is this just normal boy stuff. (Mind you, I do find some of this funny, but I try to not laugh too much so I don't encourage it)