Well hopefully he isnt positive....but you would be right to do alot of reading up on it if he is.
Speaking to you vet is a good idea although it depends how he personally feels about FIV cats. My last vet felt that all FIV cats should be PTS and certainly wouldnt recommend a NEG and a POS living together......where as my current vets feels totally the opposite way.
Found this on CatChat -
Can FIV-positives and FIV-negatives live together?
The most recent research carried out at Glasgow University's Companion Animal Diagnostics indicates that the chances of FIV being passed from one cat to another in the same household is approx 1-2%. This means that if you have 100 cats (!) in a house with 1 FIV positive cat, only 1 or 2 could be expected to become infected. Even when FIV was passed on, as in the Glasgow survey, none of the cats actually died of it.
In another survey a few years ago FIV was not passed from cat to cat in the same household at all. There is also an ongoing study by the Celia Hammond Animal Trust where FIV-positive and FIV-negative cats are living together, and regular 'spot tests' for the virus are carried out. Several years into this study no cases of transmission have yet been found.
There seems no reason, therefore, not to have FIV positive and negative cats in the same household, provided they are not fighters. If two cats in one household fight, they should be kept apart, given behavioural therapy or one re-homed, regardless of their FIV status. So, this is obviously a decision to be taken on a case by case basis, depending on the natures of the cats involved.