The premise as that since HIV / AIDS are blood born pathogens that if a boxer gets cut (which does happen frequently) the blood could splatter... perhaps landing in your eye or something of that nature... If that happens, due to the open areas around the face, it is quite possible you could catch HIV.
I find it difficult to believe that he didn't get 100 test done back in the day.
Most likely, he is on the cocktail and there are no traces... If that's the case, is it REALLY gone, or is it just the drugs.
If you get his blood in you, since you don't have the drugs, is it REALLY gone, or can it start to attack your white blood cells in your body and replicate.
I would find it difficult to take that chance.