I had him checked out by the vet last night (still can't believe how we managed getting him into the carrier, but we did), and he said there was nothing wrong. He's in good condition, his teeth are okay, and he has no signs of trauma or illness. The vet thinks it's just him wanting something new.
In the meantime, his food has been left untouched and I'm still with the same problems of him not eating except from his outside snacks.