Actually, without obvious signs like bursting pellets or profuse sweating (or a tip off), there is no way he can get caught. You don't go through any (x-ray) machines or other detection devices when arriving at US customs, in fact, normally you just walk right through without them even opening bags.
Not sure if a drug dog would detect pellets in your stomach, but maybe residue on your hands or if you fart? LOL...
LOL!!! "whew! I made it here to the airport! yes!.......uh oh, I gotta let one loose..." *Here comes a big German Shepherd barking frantically and scratching at the guys rear end looking for his prize tennis ball.