Give this a try.....
Veal Gerber 2nd foods thinned either with water or with Wiska's Cat Milk (which has Taurine and is Lactose Reduced). Take a few tsp of the baby food and microwave it about 10 seconds, mix milk or water in with a wisk. It mixes better warm. Test the temp before feeding.
Any kitten food. Or any ground-style (not sliced or flaked) cat food. (Such as any of the Fancy Feast "feast" varieties, or their regular food.) Put this through a screen-type sieve. The kitten food might not leave much residue, but you'll be surprised at the bone chips that sieve out of the adult food. Water down enough to feed.
Hills AD, if your cat tolerates Hills food.
#1 is easy to feed. #2 has the benefit of being their usual food so you know it won't turn them off. The kitten food is denser in calories and is particularly good. #3 is pre-prepared for the purpose.
Filling food syringes:
Get the 5 or 10 cc syringe either at your vet or at the pharmacy. If you go to the pharmacy, ask for a few of the 5 or 10 cc syringes used for giving liquid medicine to children. They gave me two for free. They need not know it's for a cat, if you think it will make a difference.
You can either take the plunger out and spoon food in the top, or you can put the syringe tip in the food and suck it up by pulling the plunger up. If you spoon food in, when you re-insert the plunger, point the tip of the syringe into a bowl or the sink; food *will* squirt out, sometimes with some force, and you don't want it on the ceiling (it has happened...).
Now you're ready to feed. Kneel with kitty in front of you, his tail between your knees. Grab his head from the top with your thumb and first finger on either side, just at the jaw hinge. Come in from the side, behind the fangs. Try to squirt about 1/2 cc or so at a time. Try to get it on top of the back half of his tongue, so he doesn't flip it back out of his mouth, or dribble it out. But be sure you don't squirt down kitty's throat or he might choke.
Caution: there will also be times when the plunger is hard to push. This may mean a bit of food has clogged the tip. Don't unclog it by pushing harder while it's in your kitty's mouth -- it may squirt a lot out suddenly and that will choke kitty. Unclog over the sink, into a bowl, or into a paper towel that you have handy.
Kitty may struggle a bit, especially at first when it's a strange thing. Hang on. Get help if needed.
Keep a sponge or damp paper towel ready for dribbles and spits of food. Do not wear a new white shirt!
Make sure you get at least 60cc of diluted food into your cat per day. More if possible. Break it into many small meals of 10cc each. Go slowly, especially at first. Even 1cc is one more than otherwise.
If kitty was off food a day or two before you started, start out with very dilute food (broth made by thinning any of the above recipes quite a lot). The extra water will do most cats good.