Yep, it's gross - but it's also reasonably common, so don't panic
Puppies sometimes do eat their own poop (sometimes other dogs' too) and many eat cat poop if they can get it. Not sure if there's a definitive answer as to why, but one theory is that it's the protein in the poop that is attractive. Sometimes feeding a higher quality food can help - higher quality ingredients are usually more easily digested, meaning there's less protein etc expelled as waste, which in turn means the poop is less attractive to eat... It doesn't work in all cases but it does work often enough to make it certainly worth a try (if he's not already on a super-premium kibble, that is). Raw feeding is another option - I've yet to hear of a raw fed dog eating its own poop :D
If none of the above works, there are products around that you can put in their food that supposedly make it unattractive to eat. I'm afraid that doesn't work in all cases either though.
In the meantime, and possibly as the long term solution, I'd suggest picking up and disposing of his poop before he gets the chance to eat it. Grossness aside, eating poop is a highly effective way for a pup to pick up and spread parasites and other nasties (including to you). Good luck
