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!
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