If there is cat formula available I would start him on that, it's more nutritionally balanced for cats and should help stop the diarrhea. This website
https://www.thesprucepets.com/kitten-supplement-formula-552272 offers several recipes for homemade kitten formula. Copper actually looks old enough to get started on solid foods. Get some dry kitten food and add some kitten formula, let the dry food absorb the liquid then mash it into a gruel and put it in a shallow dish for her to eat. It will be messy, and she will probably need a bath, but she'll learn how to eat solid food this way. Alternatively, you can get canned kitten food and mix some formula with it to make it into a gruel and feed that. Put another dish in with just the wet or dry kitten food, you'll be surprised how quickly she picks it up. Make sure she has plenty of fresh water in a shallow dish. She may not drink much while she's eating wet food, but if she ends up eating more dry food you'll see her drink more. Good luck with Copper!