He's probably not eating because he is stressed about his environment, he doesn't like his food, he is allergic to his food, or he has a kidney stone or some other health condition. So, it could be behavioral or medical. I saw your post about his messy litterbox. That could be contributing to his not eating. If you don't clean his litterbox regularly enough, that will stress him out. Also, buy litterbox liners. That will make cleaning it so much easier for you. And Albert will not make such a mess. Also, buy a pad to put underneath the litterbox to catch all of that litter. There are also litterboxes with high sides, so that Albert won't be able to kick litter everywhere. He may even prefer a litterbox that is enclosed. Talk to his vet and see what the vet suggests.