If you neuter to stop aggression, roaming etc, could you not release him back into the feral community? Assuming he's been around for a while, he won't pose any more of a threat than before he was tested?
Obviously if he could be homed to an indoor environment that would be great, or an enclosed garden or similar, but in case he can't, I wonder if that could be a possibility?
AFter all, how many FIV+ cats are out there already that we don't know about?