Be meticulous with the cat toilet, and since he has issues with respiratory infections, make sure the cat litter does not create too much dust. Other than that, keep weighting the cat on the regular, since weight loss is an early indicator for issues. A yearly blood count, serum biochemical analysis, and urine analysis can catch issues early.

Thanks for taking a FIV+ cat in! My mother's two cats were FIV+ (she was helping out a shelter with harboring FIV+ cats, and those two stayed in the end), and they lived for years without major issues with regular screening and early prevention if issues popped up.