Very good questions Aly.

Katie do not go and get any medication from the drug store as you don't know what is wrong with your kitty.
Take up his food but leave a little bit of water for him and take him to your vet in the morning. It could be some kind of virus like my son's cat had, even though he is not an outdoor cat. Believe it or not, the vet said that we could even bring in something on our clothes if we touched any cat that was outside or even at a friends house.
Your little one will be in my thoughts and my prayers.
Please keep us posted on how he is doing.