I have 6 feral "kids" who get premium food - the same food I feed my 3 indoor kitties.
I strongly believe that canned food is superior to dry, so I use Wellness, Petguard and occasionally Merrick. ( I'm a little concerned about a couple of ingredients in the Merrick, though). For a treat, I'll sprinkle a little dry food on top of the canned, but I never feed dry alone. The dry food I use is California Natural.
My thinking is that I'd rather spend the extra money on high quality food that may help prevent health problems, rather than on vet bills resulting from a poor quality diet. I also worry about the FLUTD (feline lower urinary tract disease)/dry food connection. It's a life threatening condition which one of my ferals had a couple of years ago. I was lucky enough to be able to trap him and get him to the vet for treatment. If I hadn't been able to catch him, and he became blocked, he could have died within 24 hours. I hope that with the majority of the diet being canned food, none of my ferals will experience that problem again.
Just wondering- have you neutered the little guy you're caring for yet? He's lucky to have someone who cares about him