well honey something isn't right with kitty, has be been checked for worms? Or had his teeth checked lately?
I don't know there could be many possibilities and I think kitty really needs to be seen by a Vet and have some tests done...that isn't natural for him to be losing weight when still eating.
let us know ok
My moms cat had the same thing happen. He turned out to be diabetic. He was drinking A LOT and going to the bathroom A LOT. At first they were thinking it was his thyroid but then they discovered his diabetes. But I agree, get him to the vet. Good Luck!