I would imagine the sample will prove whether there's anything underlying with the tummy etc linked to weight loss.
You may even find that he's had FIV for years and years, and that the 2 'problems' are not even linked at all!
Is he still an outdoor cat now?
I know certainly that when an FIV cat is placed in a new home, owners are advised to keep them as indoor only, as this not only protects other cats from having the virus passed on (through biting primarily), but it also stops them being exposed to potential viruses/illnesses from other cats.
......but, and I say this hesitantly until other peeps may give further insight, as he's an oldie and not much of a fighter now, he may not pose much of a threat to any other animals
Presumably he's up to date with his vaccinations etc with no issues?