I would call the vet and tell them he hasn't eaten. gently pinch (not hard, just enough to pull up the skin) a flap of skin on his neck, does it recoil quickly or hold its shape for a few seconds? if it doesn't recoil quickly he is dehydrated. which I would imagine if he hasn't had anything to eat or drink in days. Try adding some chicken broth to water to make it more enticing.. feed him some plain boiled chicken off the bone. IS he going to the bathroom normally?
poor guy... If i ate a pound of M&Ms i might not feel like eating for a couple days either! ha.