of the wet foods, the Hills k/d has the most calories per 100g but not many cats like it, but if they only eat a little food it's not a bad choice. Dry food usually puts on weight better than wet, but of course dry carries its own problems with oldies.
I've said this before, but a little known (in the UK) RC dry food, Senior for neutered cats over 10 yrs old, SO 28 veterinary food - vet UK do it- is eaten by virtually all of France's older cats. Mine used to eat it exclusively before their health problems and I had to watch how much I gave as it put on weight. It seems to be very palatable. If Jack has kidney issues though I'd not give much of it.