Hi again, Jane and thanks very much for your helpful reply.
Also, thank you for your patience, as I had a lot to say!
Poor Blackie! He has and is certainly going through a lot right now and I know you're worried.
It's very important for a cat to eat on a regular basis, as I'm sure you're aware; if he does not eat enough or at all, he risks developing a serious liver condition called hepatic lipidosis or 'fatty liver', which can only be cured by eating. As you can see, it's sort of a catch 22/vicious cycle. So, at this time, it's important to get nutrition and fluids into Blackie as well as you can until his mouth sore is healed enough to not cause him discomfort when he eats.
Since you're getting the antibiotic into him with a syringe in a small amount of food, I think this is the best way to also get food into him on a steady basis and fluids, also. The food the vet gave you is most likely Science Diet a/d, an easily digestible smooth food high in calories, which is used as a 'recovery' food when cats are not eating properly. It might not be this exact food, as I don't know if Hill's Science Diet prescription food (tinned) is available in your area, but ask the vet.
This is what I would recommend: Use the syringe to feed Blackie this food from the vet, mixed with a little water or 'Cat Sure' (if you have it in New Zealand); this is a nutritional supplement which contains nutrients and calories for cats who are not eating well. It's similar to 'Ensure' for humans who are not getting proper nutrition. Go in from the side of his mouth that does not have the sore, where there is an opening between the upper and lower teeth; squeeze in a little at a time, and wait for a swallow. You can encourage the swallow reflex by gently rubbing his throat. At first, give 2 cc's of the food every 20 minutes, within an hour. Then, let her rest and repeat this in 2 hours. You can slowly increase the amount of food to 4 cc's every 30 minutes for an hour.
You might also use plain jarred baby food meat contaiing no onion, garlic or any seasonings at all. He might lap this from a small bowl, or you could finger feed him. If your stores carry Beechnut Stage 1 chicken and chicken broth, this is a good one. If you get any other brand, just make sure to read the label. It should contain only meat and water for processing. No thickeners, preservatives or seasonings. You could watert this down and syringe feed it also, if he likes it.
If you can find Nutri-Cal in your pet supply store, see if he will lap this from your finger. It is a nutritional supplement which provides calories and energy for cats who are not eating properly or at all. However, make sure he is not diabetic, as the main ingredients contain sugars.
Do the same thing with the fluids. Offer Cat Sure if you can find it, or water mixed 50/50 with children's unflavored Pedialyte (electrolyte solution), with the syringe. If you don't have Pedialyte, again, ask in your pet supply store or at the chemist for an infant's or children's unflavored electrolye solution. Give the fluids uring the hours you are not giving the food. Start with 2 cc's every 20 minutes for an hour. Always go slowly and if he should cough or vomit, stop the feeding.
I believe as long as his nutrition and fluid intake is kept up, plus the antibiotics are helping with the infection, he should slowly but surely be on the road to recovery, in time. If he is able to eat soft, tinned food watered down to make it more soupy so he can lap it, as his sore heals, this is what we're aiming for. As he becomes stronger, with food and fluids, the infection should clear faster.
Ask your vet if there is anything topical you can put on the mouth sore to help with discomfort and also help it heal faster.
You mentioned he is a Tom cat and gets into fights outside. If he is not neutered, this would be advisable, after his infection is well gone and he is strong and healthy. It sounds like the lump on his face was the result of a fight, that turned into an abscess, and this could be the source of the infection. I hope the vet has evaluated this lump; if not, ask him to.
I hope all will be well with Blackie, and his recovery period might take longer than usual, since you have to help him eat and drink due to his mouth sore.
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Warmest regards XXXXX XXXXX and Blackie,