It could be just one of a few problems, or even more than one.
Here is a guide to consider....Circumcision...Uncomfortable underwear...Irritable skin condition...O.C.D...Inadequate hygene...or just simply a young boys natural pleasing sensation.
Yeah try bigger underwear or boxers for him. Maybe that will help.
Do you think it is the style of underwear he is wearing, maybe it uncomfortable for him, I believe Walmart makes two or three different styles to chose from. I hope this helps.