i wouldnt worry about it.. my nefews do the same thing.. one is 4 and one is 7. They are facinated and get it out every chance possible. N yes, they find it halarious! the visitors..not so much. lol. They take it out at the park and at home..and probley even the supermarket. Saying no to them works but not for long.. Im sure he will grow out of it eventually. if not, i pity his girlfriend/wife... dont think she would like it on her feet lol.
at the end of the day boys will be boys.