My son just cut himself accidentally by the razor blade with fresh blood on it. how chance he gets hiv if that blades used for hiv positive?

Very low risk: People shouldn't share razors, and if had been used very recently and fresh, wet blood really was visible on the blade, there might be a risk. But this is a theoretical risk that probably has never happened; I have never heard of anyone catching HIV by a razor. Presumably you know who used the razor. Is there a reason to believe s/he might have HIV? If so, immediately get your son to his doctor.