Food for gaining weight?

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?

There's a lot of brands out there that could help, like Halo, Evo, Orijen, and Wellness. They all have high calorie foods in different flavors. My cat has hyperthyroidism and renal disease and was losing a lot of weight cause she didn't want to eat any kind of food but I heard about Kidney Support Gold drops which you just put a couple drops on the food and she eats it up and it's helped her get her appetite back and she gained some weight back, so maybe that could help your cat eat more too?

The food chart at CatInfo.org might be helpful in finding some high-calorie canned foods. It's not all up-to-date but you could check it against manufacturers' Web sites. I second the idea of trying Wellness: some of the foods in the Core and grain-free lines are nicely high-calorie and low-carb.