I've dealt with a significant number of FIV+ cats in my line of work (rescue) and these are my experiences based on working with my vets and managing FIV+ cats.
First, let me me appologise in advance for a bit of a soap-box rant.. ;) - this rant is caused by years of dealing with vets and rescue people who do not stay up to date with current research on FIV by Dr Diane Addie at Glasgow University...
Unless your boy is already sympomatic (i.e. starting to show issues directly related to FIV) then he has the potential of a good, long quality of life. FIV is NOT a death sentence and PTS is NOT a neccessary option, unless your cat is seriously ill and suffering with full blown symptoms. It is also not common for it to be passed on the FIV -ve cats in the household, unless your cats fight or mate, so any -ve FIV cats are perfectly OK to live alongside an FIV+ cat and share food bowls, beds and blankets. If your vet tells you anything else, they are talking through their arse.
If your cat is NOT a fighter, then the biggest risk to an FIV+ cat is infections from other cats, because obviously his immune system is compromised. i.e. things like Flu, bacterial infections etc. You need to be extra vigilent with sniffles and sneezes, bumps and grazes and if in doubt about anything, VET! IF your cat IS a fighter, then he can pass on FIV in bite wounds. He can also pass it on through sexual contact.
There are a number of things on the market which mean you can game-park/secure your garden, things like Cat fencing but they are expensive.
If he he is happy with "restricted" outdoor access, then continue as you are doing by letting him out when you can supervise at home. The less stress you can give him, the better.
I'm happy to help by PM if you want. I'm not a vet, but I have many years of working in cat rescue, as well as dealing with FIV.