One of the best things I've found to make a sick cat eat is 'chicken broth' baby food. Its easy to digest, smells good, and you can give it to them on a spoon. Also, rather than giving him water, give him pedialite... the electrolites will help hydrate him fairly well. What I do is get a glass full, room temp, and stick a straw in it... cover the end of the straw so your holding liquid in the straw, and give it to them that way.
If he can't smell something, he generally wont eat it. Cats are like that. Let him rest, keep him quiet, and take him to the vet as soon as you can. Odds are though the vet will be as stumped as you are without a ton of blood tests. You might take his temperature and see if hes running a fever yourself.... allerigic reactions don't generally make them run a fever...
Good luck! I've got a ton of felines at home that are gonna think good thoughts along with me for you.