Hi Lainey,
I also have a runt girl, yet I have to say that I don’t add up some special food for her. The vet also found out that Milly had parasites on the 5th day after I got her. I added Nutri-Cal for the first two weeks and tried to feed her kitten formula, but she wasn’t interested in both.
I fed Milly exclusively on RC Babycat, but she ate so little. She lost appetite during her medication and Whiskas Junior wet was all she wanted to eat. Though Whiskas might not the best option for cat food, it helped her going through the problem. She’s now been gaining threefold. On whiskas wet, mixed with RC Babycat dry. I’m still on the way to get her back on better diet. But i’d say, that “as-long-as-he-eats” policy is indeed good. You can always fix the diet later. Now he really needs to recover, and if he’s losing appetite, Hill’s a/d might help as a start. But maybe you gotta have to venture a bit through some brands of cat food to find the one your cat loves the most. Just pay attention on the ingredients details, and beware of some coloring and preservatives.
Good luck!