How much are you feeding him? Too much food would result in too much poo.

One thing I am curious about though, is why he tries to eat poop outside all the time, but when he does it in the basement, he leaves it all there
I agree with lv4dogs, he is probably eating someone else's poo outside. In the basement it's always his, therefore he does not eat it. Is there cats that would be pooping in the yard? Dogs usually like cat poo.

Have you ever considered crating him while you are away? At least for the shorter times.