Since the gentle method does not work, how about becoming really furious and forbidding him to do it? What he is doing is not so awful in itself, but socially it can be a disaster for him. You have to save him from that with whatever method works.
my 4 yr old son will not puttin his hands down the front and back of his pants and then he ***** his thumb... its nasty i have tried everything and i dont know what to do please help....
*~*Lost*~*
My 4 year old son has done this a lot too. I think it is a comfort thing and I don't even think he's fully aware he's doing it until I tell him to pull his hands out. He does when I tell him. I think they will outgrow it, especially once they're going to school full time, as the teachers won't allow that and the kids would make fun of it. That usually stops it if it hasn't stopped by then!
Some little boys just do this, it feels good and becomes a habit that is hard to break because they're too young to care what is socially acceptable. (But you already knew that, didn't you?) I know two mothers who had to deal with it, and both put their boys in tighter type pants with a snug waistband especially when in public (and preschool) to discourage it. In other words, jeans instead of sweatpants, shorts with snaps and zippers instead of an elastic waistband. It's a pain for you as far as shopping for clothes, but it makes it more difficult and uncomfortable for them to keep their hand in there. At least these days there are lots of pants and shorts with the waist size adjusters inside, so you can make them fit without a belt. Hope this helps~
If it is any consolation, I don't think it is a "sexual" thing, but a "comfort" thing. One of our grandsons did this when he was upset or tired. But, of course, this is not be acceptable behaviour when he goes to school so your son will need to break himself of this habit. Perhaps others will have some suggestions on how to do that (besides nagging him everytime you catch him "in the act'). I wish you the best ....