Cooper, my new yorkie eats his poop!

I was going to start tomorrow giving him the pineapple; today during his walk he started eating another dogs poop! Now thats just disgusting! So I'm pretty sure now, he will eat Lulu's poop too. Why do some dogs do this?
Cooper is 1.5 years old, neutered...He marks everything! I'm starting to believe this is the reason he was given away. Is there anything I can do to stop this at his age?