They all do it, even bigger kids mostly its adjusting ' that something , I have seen all males doing it it seems to be a popular' game"
This is normal, but just teach him to not do it in public places and teach him that his bedroom is the only place to do it if you approve of it.
hi my son is 6 has been doing that for the last 3 yr and doesnt matter what i say to him he still does it