What is a good cat food to help him gain weight?

My cat is pretty thin from hyperthyroidism and needs a cat food, both dry and moist to help put some weight on him. The vet gave him a liquid vitamin for twice per day, but I want to feed him a high quality food that hopefully does not contain added salt or fish (any seafood). I am hoping there is a food for very active cats available, so to give him extra calories.