I don't what your prescription dry food is, but it may be crap [Science Diet is the only prescription cat food I know of and it is horrid, horrid food]. So are you sure he needs it as opposed to a high quality cat food like Blue Buffalo, Pinnacle, Orijen, etc?
I steer clear of wet food unless its of a high quality brand like the ones I listed. My parents give their 13+ year old skinny guy a teaspoon or so of tuna fish in oil once a day. It's worked great for him, he's not fat but he filled in and is healthier than ever.
Feeding your cat poor quality food doesn't give him Enpugh nutrients to store. So you may be feeding a ton of something low quality but since it's mostly garbage, it'll pass through. Versus a high quality food that you can feed 1/2 a cup a day and be battling with obesity, lol.
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I used to feed what ever to my cats. Until I heard a nutritionist give a talk about what goes into pet food. He said the first three ingredients make up the bulk of the kibble. Corn, corn by-products, etc are just junk as corn is not that digestible. Cats are obligate carnivores, meaning they really can not digest plant matter.
If you can, get a pet food that also does not have any grain in it at all. For example, I am feeding Taste of the Wild or Solid Gold (I think that is what it's called - it is in a gold sparkly bag). Neither has grain in them, Meat is the first ingredient. And not a by-product either. It has blueberries, carrots, things like that.
You don't have to feed as much, or if you free-feed, the cat will eventually eat less because they end up being able to actually use what they are eating.
And if you try the tuna juice, give them the juice that is water. If you do the oil you could give too much to start with and give the poor guy the runs...