This is not uncommon in young dogs, but it is a behavior that needs to be interrupted and resolved. If you do see him eating poop, react in a distressed rather than angry manner: "oh no!" This is more likely to get his attention in a positive way. Follow this up with "leave it" which is a very important command overall. Always follow up successful obedience with positive reinforcement, preferably a high value treat and really excessive verbal praise. Use a high-pitched exaggerated positive tone, the way that you would talk to an infant. It might seem silly but it is very effective. Dogs are so responsive to tone in our voices and positivity in our body language. I highly recommend this trainer on YouTube. Here's a video specific to leave it, but he has videos on basically any training topic you can imagine. I find his teaching accessible and useful for core skills.