About a month ago, I brought home a new kitty. He's about 1 year old and has been eating a mix of the food he had at the shelter and the wet food I feed my other kitties. He's been pretty good about eating until the other day, when he had some explosive puke. Since then, he hasn't really eaten much. I even went back to giving him only the food he was used to from the shelter, but he won't touch it. Is this something that might go on for a few days and then right itself? Or should I take him to the vet ASAP? Any signs that it's something more serious than an upset tummy?