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The FeLV vaccine will not give a cat a false positive for FIV, but the FIV vaccine will. I wouldn't recommend ANY cat get the FIV vaccine, it is not effective at best, and will cause the cat to always test positive.
If you keep your boy indoors, he does not need the FeLV vaccine.
A cat can't "turn negative" if he has FIV. If he was tested prior to six months old and showed positive, and then later as an adult tested again and showed negative, one of two things has happened. Either he was tested too young and was still carrying his mother's antibodies, which is not uncommon, or the first test was wrong.