Hello KittyMommy
I do not know a lot about FIV, only that it is similar to our HIV virus. Loss of appetite is a common symptom. Are there any other symptoms like weakness and fever? Is he living indoors?
If he is living indoors, and he actually is in the later stages, he still could act normal, as the virus does not have any serious effect on him: The virus just weakens his immune system, leaving him defenceless,so even the simplest cold may prove fatal: staying indoors, the chances of catching a cold or any other disease are minimum.
My girlfriend's kitty, a 9year old female, was diagnosed with FIV. She kept her indoors most of her life, and with the proper antibiotics and medicine, the kitty lived to 11,5 years.
However I am not a vet and I cannot give you information which could jeopardise the life of your kitty. You can find more information on FIV here:
http://www.catconnection.net/care/fiv.htm