If he's pooping and active than I'd guess he's probably eating something. Is there any chance he could be eating/drinking when you're not looking and you just don't notice?
Foods like greens and fresh grass have a lot of water in them. If he's eating a lot of grass in the yard, he may not need/want much other food or water.
Keep a close eye on him. If he stops pooping or starts to act any differently I'd get him into the vet.
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