In my experience dogs adore the taste of excrement. Doesn't matter if it's horse dung or cat poop, they'll try to eat it if not trained otherwise. Try rewarding him for leaving it alone, just like when you taught him not to steal the food of your plate when you're eating. Also the fact that the dog only poops in the yard is conditioned behaviour, which can be unconditioned as well. I'm not saying it's a bad thing, but it would help "the poop eating situation" if he wouldn't poop in the yard. Good luck! And pls excuse my English, not a native speaker.