Jen, while vomiting could be a symptom of FIP, as with any disease, it is not common unless other systemic effects of FIP are causing it. Generally, it is poor appetite, unrelenting fever, distended abdomen, depression, weight loss and poor coat condition that are the classical signs of FIP.
One solution to attempt is ceasing the soft food and gradually starting him on an adult maintenance dry food. Any cat can develop a sudden sensitivity to any food, and this just might be the problem at this point. Since he is vomiting more than normal, you can't risk dehydration, so ensure he is getting plenty of normal water intake on a daily basis. Also, if his stools are loose or has diarrhea, opt to get a fecal done by your vet to rule out parasites or bacteria.
As for the thermometer, you can get a digital rectal thermometer in any grocery store pharmacy dept, or pharmacy alone. Keep it seperate from the one you use for yourself and family. To keep it properly cleaned between uses, simply use a gauze or cotton ball dipped in rubbing alcohol, this acts as an antiseptic.
In the event you've never taken kitty's temperature, be sure you lubricate the tip of the thermometer with vaseline (regular, yellow type), then insert only 1/4 - 1/2 inch, wait for the thermometer to register or beep and record it. Normal feline body temperature is 101-102.5 Â° . Anything significantly higher than 103 demands a vet visit. Remember, you cant accurately gauge a cat's temperature by the cat feeling warm to the touch or dry nose, these are inappropriate methods of gauging core body temperature. Lethargy, as you noted, does warrant a talk with your vet/possible exam........................Traci