If you want to give him a good food, Taste of the Wild--Canyon River. It's all I feed my cats. I feed my dogs Taste of the Wild--Pacific Stream. Remember cats are carnivours, they don't eat grains, unless we people give it to them. They do eat grass from time to time for medicinal reasons.
When my 18 year old was around 2 she got "something" no idea what it was. She wouldn't eat, just laid around, didn't want to go out. Took her to the emergency vet (waste of time and money) my regular vet put her on IV drips for 3 days and did all kinds of tests just like your vet did. NOTHING! They let me take her home that weekend to see if I could get her to eat. Eventually she did, and got better. We still have no idea what happen. She was sick for about a week. She turned 18 in March and is an evil old woman, lol. But we love her
ETA----I have 2 BIG boys as well. The 14 year old is 17lbs, the 2 year old is 12lbs and not filled out completly. The boys are "solid" they fly through my house and over, and under furniture, beds, kitty climbers, etc. Not an ounce of fat on them. My vet told me a longtime back that you have to keep an eye on the big guys for signs of diabetis. So far so good on mine. Positive thoughts, yours will be too!!