I remember this happening to Taz. I worked years to get him to lose weight, then suddenly I was working to get him to gain weight.
Is Scooter eating good? My vet said to give Taz anything he'll eat. "Kitty junk food" as she called it. I began giving him canned food, which he never had before because I didn't like it. LOL Although Taz never really gained any weight due to his kidney failure, I hope you can get Scooter checked out to make sure there isn't a "hidden" illness. And I hope he will begin to gain weight. Hey, don't we all gain a little weight over the holidays? Maybe Scooter can too!