its natural. children love exploring things. for most of your childs life his willy has been hidden inside a nappy. when he starts wearing pants he discovers it, and whats more it feels nice when he touches it. Please dont tell your child that it is dirty or shameful, this will only serve to mess him up when he is a teenager and having proper urges to touch it. Insted tell your child that it is fine, but that people dont need to see him do it. if he wants to do it he can do it in his room. But if it is more of an absent minded fiddling, just distract him. dont even mention his willy and say insted "come and play with the cars" or "draw a picture". give him something else to do with his hands!!!