Garfield needs to gain weight

A couple of months ago I had to take Garfield to the vet, because he had lost alot of weight, and was dehydrated.
Well, he had a kidney infection, so he had to stay overnight at the vet on an IV, and be given lots of anibiotics.
So I've been trying to get him to put some weight back on without any luck.
He used to be fat, and after his kidney infection his ribs, and shoulder bones, and hipbones can all be felt.
I am feeding Fancy Feast canned and Natural balance dry free choice to all of the cats. He gets a whole can to himself every morning, but if I try tom give him anymore canned he gets really loose stools. Any other suggestions?

You could try a little kitten wet and/or dry mixed in over the short term. It is higher in calories and more dense with nutrients b/c of a growing cat's needs. I did that with a skinny/underweight cat I rescued from the pound... I gave him small amounts - about 50/50 initially. You want him back to a normal weight slowly.

You would have to gradually wean him off of it and transition him back to his normal food. In the future, I would also perhaps change the ratio of wet to dry you are giving him... for instance, more wet, less dry, but still at a caloric rate that helps him maintain a healthy weight.

Re: soft stools. Some cats cannot handle too much wet. You can try a digestive enzyme/probiotic product, too. That will help him process his food. Something like Benebac, Probios, Animal Essentials, Prozyme.... etc. Search here to get some recommendations. It wouldn't be a bad thing to give to all of the cats. Mine get it every day.

You could also break up that wet food serving and give him half in the am and half at dinner.. however you feed.

Thanks for the suggestions.