Switching a cat's food.....

My employers have a 2yr old male, DSH, who is underweight. Not quite skin and bones, but looking at him from above, there is too much of a dip along each side. I'm going to run a fecal sample to the vet to check for worms, but he's just been a few months ago and he's an indoor cat, never goes out and is the only pet. I'm pretty sure it's just that he's not getting all the nutrients he needs, because he's on grocery store food (meow mix, I think, can't quite remember). I've asked if I can switch him to a better food, and they said yes, so I'm going to go pick something up. Does anybody have a food reccommendation that is nutritious and will build body mass?
This is new for me.....my cats have to worry about staying slim, because they'd eat till they split if I let them.

I use natural choice kitten mixed with solid gold for Zoey who doesnt keep wt on well at age three ...

Chicken Soup for the Cat Lover's Soul is working great for my cat who is recovering from anorexia and really was practically skin and bones.

Chicken Soup is good, but I'd vote for Innova HealthWise if you can get it. It's in the same price range as Chicken Soup, (maybe even a bit cheaper), and I've found my cats take to that better than the Chicken Soup when I was testing for what to feed them in the past.
Here's a link to the Innova HealthWise Cat & Kitten food information.

It's not available here locally, so I always got it it PetFoodDirect. Here is their link.