I always add some water to my cat's wet food and mix it in. If instead of water, you could mix in some all meat baby food (no onions or garlic) to his food, that would add some extra calories.
I used to use Iams Maximum Calorie, but sadly they don't make it anymore. I would see if you can get some of the a/d cat food and add it to his regular food regimen.
When you go to the vet, ask them to check his thyroid too, if it hasn't been tested recently.