Draw length is too long, stance is too open, too big around the middle, maybe locking out his bow arm and he's probably leaning back to boot. Try him with a bow at least a couple inches shorter in the draw. Have him move his back foot forward to turn his shoulders more to the target. Keep the elbow unlocked with maybe a slight outward bend.

If all else fails, he can buy a chest protector to keep his stuffin's out of the string.