changing cat's diet to add calories

I have two rescue cats, a girl age 5 and boy age 8.  They are short-hair domestic plain black cats.  About three months ago, the boy developed a serious infection in his anal gland, it eventually ruptured.  He stayed at the cat hospital for a week and got antibiotics, treatments etc.  Now he is much better. However, he still has not gained back all of the weight he lost while being ill.  I read a bunch of posts about how supermarket cat food has very little nutritional value, so for the past three days I've been gradually phasing in grain-free high calorie wet and dry foods.  He will eat a few bites of the new kind, but then walks away disinterested.  He's a grazer, so that's not too unusual, but I fear he's not eating enough.  I have tried Blue chicken, turkey and Halo chicken thus far.  He's not crazy about any of them.  I also put out a small bowl of kibble - Wellness Core original formula, and today I tried Blue Diamond grain-free adult chicken kibble, which looks identical to the Wellness brand btw.  He hasn't taken to either brand of dry food.   The girl cat is a little less picky but I'm afraid she's not getting enough calories either.  I understand that better quality food has more calories so they can eat less, but I'm not sure they are eating enough.  I put a few tablespoons of their old wet food (friskies) in their bowls as well, which they do eat.   The boy cat seems to be sleeping more than he used to and I've noticed some dander on his back.  I'm concerned this regimen isn't working.  I don't detect any change in their litterbox output and neither seem to be sensitive to the new food with one exception - the girl threw up a little un-processed food on the first day.   Should I stick with this routine or keep alternating brands and flavors until I find one they like?

Canned Friskies is fine. It's the cheap dry foods that are full of grains and artificial colors. I'd keep on with canned Friskies and just see if maybe he'll eat a little more than usual to help him gain the weight back.

For a high-calorie dry food that most cats seem to like a lot, try Purina ONE kitten food. It's not top-quality but it's decent, and my cats will just do anything for it. Since it's kitten food, it has more calories and fat than adult food.