D.C.
Giving him the benefit of the doubt----maybe his underwear doesn't fit right. Perhaps he needs boxers so he can hang freely (LOL at 6 yrs. old?) Or maybe snug jockeys that hold him in place,once he gets it all adjusted!
But then, maybe he just likes to feel it! If that is the case you have to make him understand that other people don't like to see him playing with himself. He may be doing it unconsiously now and needs something to remind him until he breaks the habit. But break it he must. I hate to see a grown man adjusting himself all the time!!!
All that said, I'm sure he will quit. Don't despair.
God bless.
D.