Hello, my name is Dr. Kara and I have over 20 years of experience as a veterinarian.
I am so sorry to hear that Barney has been diagnosed as FIV positive.
FIV positive cats often have painful secondary bacterial infection and ulcers in the mouth. We cannot cure these lesions but we can make them more comfortable by controlling the infections with a thorough dental cleaning to reduce the bacteria population, oral antibiotics, and oral rinses like CET rinse to decrease bacteria in the mouth. I also recommend pain medication such as buprenorphine.
These are temporary control measures but they are worth trying if you are not ready to euthanize him yet.
I also recommend a complete blood count and biochemistry panel to look for underlying organ failure. FIV cats will often have kidney failure at a young age which leads to mouth ulcers, a poor appetite and anemia. We can help control the symptoms of kidney failure with specialized diets, acid reducers such as Pepcid ac (famotidine), fluid therapy, and other medications based upon whether he has secondary high phosphorus levels or high blood pressure.
In the meantime I do recommend fluid therapy now so he doesn't become dehydrated, an appetite stimulant such as Mirtazapine or Cyproheptadine and feeding soft, easy to eat canned foods.
Best of luck with Barney, please let me know if you have any further questions.