What food are you offering him?
I have found when dealing with serious illness in my cats over the years, that when they are sick they aren't always tempted by the food they normally eat, like kibble.
If you think that he is enjoying being syringed the baby food, how does he react if you warm the jar and offer it by the spoonful? I've found mine will lick it off the spoon and if I have to spoon feed the entire jar then that is what I have to do. But if he's enjoying that then he might well also enjoy some boiled ground chicken. I get the ground chicken fresh at Whole Foods, then boil with a lot of water so it is soupy and they seem to find that attractive. Others prefer ground beef cooked that way. Try lots of things and see what tempts him...anything to avoid returning to the vet for his sake?
Try canned tuna, cooked shrimp... some people find strips of cooked steak tempt them... sandwich turkey. The idea is to get him eating anything, you can worry about dietary needs later on. With one of my cats I tried everything and then found out the wet form of Meow Mix tempted him. Disgusting stuff but he ate it. When he was well again we weaned him back onto a better cat food.
Hoping the mirtazipine works, it helped one of my cats during a rough time too...