Elderly cat information?

I have a wonderful cat named Tuxedo (but he has always been called Big). He is FIV positive (he contracted the disease before there was a vaccine for it). Im concerned because his weight is steadily dropping. He feels so very thin. His mood is good and his appetite is healthy, but I am still concerned about him losing so much weight. His last check up (about 4 months ago) showed everything was fine with his bloodwork. I guess what I am wondering is if anyone has any good ideas on what I can feed him to boost his weight. Most of the senior feline diets are low in fat and high in fiber...and seem to add to his severe weight loss. He is my heart, and any advice is appreciated.