It is always wise to follow the advice of your veterinarian. Whith that said, I have a friend whose FIV cat has lived more than 10 years after being diagnosed with the disease. She has used homeopathic remedies with him since his diagnosis. You might ask your vet about this.
By the way, I am so sorry to hear about your cat's illness. My heart goes out to you. I pray that he will live a long and healthy life.
And, I noticed in your other post that your vet advised that your cat be castrated. I agree strongly with your vet. Castration will reduce his stress level, it will make him more satisfied with staying home, and will make it less likely that he will fight with and infect another cat if he gets outside. Castration is considered a normal thing to do here in the USA and we are surprised when we hear about house cats that have not had the surgery!