Is h also drinking a lot? More than normal?

Does he feel hot to the touch?

Eating a lot and not gaining weight is one sign of hyperthryoidism. Or of diabetes. Or of a lot of other things. It is not really something you can determine, other than to decide if he needs a vet visit. Then they will do blood work to see what is happening.

My Mr. Amber Cat had over active thyroid, and along with eating like crazy, he was quite warm. I felt like I was next to a boiler when he snuggled up against me. Ruffles had overactive thyroid and hung out by the water bowl. Never noticed her being hot or eating a lot.