If letting your son go to the store and pick out his own underwear won't help, why not try something else? My 3 year old son is very small, and even size 2T - 3T underwear fall off of him. We switched to diaper covers instead. {No pink - I promise!} It made a world of difference, and now his underwear are no longer uncomfortable and falling off, plus he loves the cute, fun designs on his "underwear" now. The dollar store has some really cute diaper covers for boys, and they're a soft cotton material so they'll be comfortable. Good luck!.