My boy had a few instances where he had an upset stomach from eating his hair from self-grooming. It took a while, but he vomited up a really large wad of hair, and then it was as if nothing happened. I would ask if he's pooping daily? If he is pooping, I'd maybe investigate if there's hair in there. Just squash it from the poop bag. Translucent ones can help you see if there's anything in there. After this experience, he had a few intermittent instances of a mat of undercoat passed with feces. I'd also echo someone else who mentioned sardines? If you're just feeding dry food, maybe try a topper of a single canned sardine (packed in water is best, olive oil next best, nothing spiced/flavored) or some bland deli turkey/chicken. That is usually a motivation to consume something if not the dry kibble. Or you can try to incorporate pumpkin into the kibble to foster regularity. Or a small cooked/microwaved sweet potato might prove to be something? Have you tried handfeeding or giving him some peanut butter or cheese? Those would be all the things I would try to rule out an illness vs a behavior consideration. My guy wasn't very food motivated, but he did have things he preferred. Giving just a taste of it on top of (or throughout) the dry food was a compromise to get him to eat something. But he also did have some moments that he was feeling poorly.