I need help finding a food for an older male cat that will help him gain weight. We currently feed him a mix of Innova wet food with Fromm's duck kibble and I spread a layer of Zuke's chicken treats over the top. I've been playing with mixing in an 1/10 of a cup of Science diet's urinary track kibble because it's higher calorie count, but I don't really see any difference. He's been through all sorts of tests at the vet and they don't have any answers. He's always been a lean cat, but the last year weights been dripping off of him. He balks at the smell of the two high calorie additives I've been able to get my hands on.. the GNC one they sell at petsmart and one from my vet I don't remember the name of. Any suggestions?