He should be seen by his veterinarian. There are a number of things that could be going on: Intestinal blockage, organ failure, thyroid disease, food allergies, or it could simply be hairballs. The veterinarian will likely recommend bloodwork and/or x-rays to properly diagnose the problem. If the cat is not eating, or is vomiting any food that he does eat, he should be evaluated sooner rather than later. Here's hoping it is nothing too serious. Good luck!
