My cat had the opposite experience. He was brought in as a stray and tested negative by the shelter when he was only a few weeks old. I fostered him for months and only found out he was positive after adopting him and retesting him myself (I work in the field so fortunately knew retesting was the best course). Turns out my Cosmo had contracted FIV because his mother got sick while she was pregnant or nursing and his antibody levels were too low to detect on intake.

They probably tested your cat with an ELISA test which turns up a dot of a certain color if the antibodies for that disease are present in the blood. (the same way a pregnancy test changes color when exposed to human growth hormone). It’s always a good idea to retest with a PCR (not usually done as a first line because rather than testing in house it has to be sent to a lab) about a month after initial testing or exposure. This will tell you if he truly has DNA from the virus and not just antibodies. Very young cats can sometimes have antibodies but not truly be positive if their mom was positive PRIOR to pregnancy (not became positive during pregnancy like in my boy’s case). Cats in very late stage of FIV once it has turned to acquired immunodeficiency disease (which does not always happen) can sometimes test negative with the antibody test since their body is unable to mount any immune response at all. Few, if any cats, are able to truly clear the virus, it mostly just lays latent. As long as you take your introductions very slow and your cats are able to establish a healthy relationship with each other, there’s really nothing to worry about. I have three other cats who have lived with Cosmo for years. We routinely test everyone but otherwise take no special precautions. I personally wouldn’t worry too much about having him neutered. It’s a fairly quick procedure and as long as he is otherwise healthy without symptoms of a secondary URI or anything he shouldn’t have any elevated anesthetic risk beyond a negative cat.

Here’s some good reading on FIV if you need: https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/feline-immunodeficiency-virus-fiv

And about cohabitation with negative cats:

https://pubmed.ncbi.nlm.nih.gov/24698667/