I'm so sorry to hear about your cat's illness. I had a cat that had IBS her whole life, and the poor thing was just miserable without her meds (prednisolone injections and occasional antibiotics). Nutrical is excellent for putting weight on cats that are in health crises. Most of them just love the taste of it, as well, so much so that they will lick it right out of the tube. It's sold in most vets' offices, however it's also available at places like PetSmart, and it's considerably cheaper at PetSmart. My IBS kitty has since gone to the Rainbow Bridge (this was some 15 years ago when I had her), however I do cat rescue and I still use Nutrical when necessary because it's such a great supplement. The last time I bought it at PetSmart I paid under $5 a tube for it, while it was almost $10 at the vet's office, so my suggestion would be that if you decide to use it, purchase it at a retail pet store and save yourself some money.
Ghilly
there are some high calorie wet foods available by prescription. i was prescribed a 'science diet' variety for one of my cats. you can also try baby food or a high calorie food supplement that you can probably find at your pet supply store. but, I'd ask your vet about a prescription for some of the high calorie food.