Is he still not eating? Is he picking at his food or just not eating at ALL? It's important to call the vet as soon as you notice the kitty isn't eating, please use me as an example. My cat stopped eating but I figured she was eating when no one else was around since she has an aggressive food loving sister and I didn't notice something was wrong until I pet her and realized she was getting really boney. Long story short she now has a feeding tube due to fatty liver disease (you can see my post about this in the community). I'm not saying that is going to happen, your cats symptoms sound different than mine (mine wasn't sneezing or anything) but cat's bodies aren't made to process their own fat when they aren't eating, and when they DO start processing their own fat it causes fatty liver disease.
If your cat is not eating or eating significantly fewer calories than usual, this is an emergency situation...like Alison said cats go into a condition called fatty liver disease real quickly and this can be fatal without treatment asap
heres a link to the post Alison mentioned, please see a Vet!!!!
http://www.medhelp.org/posts/Cats/Cat-with-fatty-liver-disease-and-feeding-tube/show/2350338