You asked earlier about Karo or molass. Its sugar sources, good to use if they need to be perked up. I dont think they have anything to do with constipation.
No poop. I switched to a different KMR, maybe that will help. Sick kitty is not doing well. Refused to eat thickened formula completely, so I switched to regular, but I am barely getting 1cc down every 1.5 hours or so. He walks around a bit and is alert, but after a great day yesterday, is not doing well.
Have you tried giving the sick kitty some pedialyte? If he's not taking formula, push the pedialyte, and keep trying the formula as well. They go downhill so quickly when not eating. He really needs to stay hydrated. I am puzzled as to what the issue is. Has he finished his antibiotic?
It looks like he is hungry but a bit frightened of the bottle or syringe. Try putting him on a towel on a table, so he doesn't slip and holding him by the scruff of the neck. That will calm him and you can slightly tilt his head back as you hold him and introduce the syringe in the side of the mouth while you squeeze gently. I have found this to be the best way with reluctant kittens. Probably after a few days he will welcome the nozzle. I usually use a syringe rather than a bottle - they seem to take to it much more easily.
My little guy was, and still kind of is, constipated. I found that giving him lots of water, watered down bottles/syringes or just water in between feedings, and rubbing his little tummy for a bit helps a lot. Have you tried burping your little guy too?