Alexa, Try buying the jars of strained Baby Food Meat like Chicken, Turkey and Beef, It is in the Baby Food aisle of your food store. If this works you can then try adding a little mashed up cooked rice to it and/or mashed up cooked egg yolks. The vet also sells a high calorie food supplement that comes in a tube which you could put on his paw to lick off if he doesn't lick it off a plate. It's called NutraCal here but might have a different name where you live.
Good luck. Do keep your vet informed of how he is doing as well or certainly bring him in again.