
If this is a recent change I'd look at diet - some foods seem to trigger it. Other possibilities are problems digesting their food, in particular digesting protein (I believe this is partly why the pineapple trick works for some dogs - not through the taste, but because the bromelain enzyme in pineapple digests protein very well so helps the dog digest it); there could be other deficiencies too.
Or, some other medical problem causing his appetite to spike so he's looking for whatever he can find that's vaguely edible - I've had this with my oldie, eating poo to satisfy a crazy appetite. With her I don't know what's causing it (and it's not constant, it just spikes sometimes), but she is old and tired so not up to any more investigations now.
Paige also started doing it recently - she needed worming and has stopped since that was done.
So there's no one cause or one answer - one thing that is pretty constant is their response to how we react. So that is important - do NOT get angry or try and shoo him off as he could start seeing the poo as more valuable and try harder to munch it before you get there. You could also trigger anxiety over it and end up with a dog that tries to hide it - I think that's how Tia started originally (my brother used to kick her for pooing indoors, I found her recently and she never ate poo before that). Best thing to do is distract with something really high value and reward for coming away or not going over to it to start with, and get a solid 'leave' going.