There's something that's concerning me here that nobody seems to have noticed.
The reason that FIV+ pusscats need to be indoor is not just about controlling any risk of their passing it on.
It also has rather a lot to do with protecting them from potential sources of bacterial or viral infection - namely, other cats and their effluence.
I'm not saying don't or anything but just bear in mind - is where you want to take your cat safe for him as an immunocompromised individual?
There are window boxy things I've seen photos of for people that don't have gardens but want their cat to be able to get a bit of fresh air - maybe that's worth looking into too? Bit like a cat balcony, I s'pose. I'm saving up cash to get net frames to put into my upstairs windows so that I can have them all wiiiiiiiiiide open and not worry about anyone getting out - that might work too.
Please forgive me if it sounds like I'm trying to teach my grandmother to suck eggs - I am acutely aware that any or all of you probably already know this stuff and then some, but I just wanted to make sure it got said.