How do I stop my adult dog from eating his own feces?

Pick it up immediately, so the dog doesn't have the opportunity to eat it. You can also add canned pineapple or a purchased product called Forbid to his food. Both make it undesirable to eat.

Clean it up before he has a chance to eat it.

By cleaning up after him each time so he can't access it, for starters. Teach him the leave it command, which may one day save his life, should he pick up something toxic.

Training and get it a better dog food. Usually it's because they are lacking something in their diet. If that doesn't work bring it to the Vet

you train and housetrain it

There is a product called For-Bid, that may work. You put it on their food, and it is supposed to make them not want to do that anymore.

Pick it up....

Dogs eating there own feces can be sign of a medical problem so its worth a visit with your vet to rule that out.

Pick it up right after he goes....before he has a chance to eat it.

You can't. Dogs are brainless animals and will eat anything they see.

Cats on the other hand...