My name is***** and I am sorry to hear that your daughter's cat is so sick! Not eating is obviously a sign that he is very ill and I would consider investigating further into the cause. You can consider an abdominal ultrasound, urinalysis, and a GI panel (to look for inflammatory bowel disease). If the vet you are currently seeing can't figure out the cause of his lack of appetite, she may also consider taking your kitty to a different vet for a second opinion.
When cats stop eating they often quickly develop a condition called Fatty Liver or hepatic lipidosis. This is likely what is happening to your kitty and they can fully recover from it with good supportive care. Care includes force feeding to support the liver but there are different ways to go about it than with a syringe. Many cats require a feeding tube placed but this is actually very comfortable for the kitty and much less stressful.
Here is a link to a more in depth discussion on fatty liver:
For your kitty tonight-
I would stop force feeding him. He obviously is tortured by this procedure and you are only making him more miserable. Also, if he is panting now or having labored breathing, he may have inhaled some of the food into his lungs.
In the morning, he needs to go back to your vet or to a different one to consider some of the above diagnostics and to have a feeding tube placed. This will make feeding him so much easier. They also can eat normally with the tube in place, so as soon as he is feeling better he can eat on his own.
Does this help? Do you have any other questions?