My 14 year old son is STILL getting poop stains on his boxer shorts? its horrible and embarresing for me & him?

im sick of it, he always does it, every pair of his boxers ALWAYS have a skid mark on them, ive told him before, and even tried embarring him in front of his 8 year old cousin by bringing them out and saying "look liam, skid marks AGAIN this has to stop" but he never stops it. what should i do? do you think theres a problem here? i mean he is only 4'5 for his age so thats small, but surely that wont be a problem? would it ....?