I'd recommend antioxidants and Omega-3 fatty acids, at a minimum. There are about a million antioxidants to choose from; vitamins C and E are common ones and do a good job. The herb astragalus is a great immune booster. Here are some more ideas and details:http://www.littlebigcat.com/index.ph...forcatsanddogs
Of course, a great diet is the main foundation. What are you feeding him?
Stress management is really important for these guys. Flower essences are excellent for that.
Oh, I almost forgot, here's an article on Living with FIV!http://www.littlebigcat.com/index.ph...=livingwithfiv