TJ has decided to take up a new habit, eating his own poo!!

He doesn't do it if I get it up right after he goes, but if I don't catch him, he starts to eat it. It is really gross! I mean, I don't want him licking my face now! How can I train him not to do that? I tell him "NO" when I see him doing it, but that just isn't cutting it! Does anyone have any advice?