It could be soooo many things only a trip to the vet will really help him. My only advice is for you to make an appointment with the vet as soon as you can. When a cat stops eating we are talking serious business here. Cats, especially babies and indoors are little pigs, so it is odd and dangerous if they refuse to eat.
Let us know how he is doing and what the vet said. Keep us posted.
A visit to the vet is DEFINITELY in order here. There is no way that anyone here can really hazard a guess at what is wrong with him. Please don't waste time, though, because cats are EXCELLENT at hiding the symptoms when something is wrong with them, and very often, by the time they DO appear to be ill, they are VERY ill. Please let is know what you find out from the vet's examination.
Ghilly