From his Wikipedia page
It sounds like there was some tampering involved when he got retested. Though, foul play aside, it's entirely possible that him being on ARVs for so long reduced his viral load to undetectable levels. He was already nearly at that point well before 2007. Rest assured though that he had full-blown AIDS. His own mother came out and said it, the commission had previously found him to be HIV+, and his own physician confirmed their findings. Tommy was probably just in denial (especially since he wanted to keep fighting), as was his wife, who didn't want to believe he had it.