Not specific amount, but think for yourself. Would it be reasonable to travel to the US with $100 for a two week trip? Probably not.
What all the ties does he have to the home country? Does he have a well paid job there, property etc? That's more important. Also, on DS-160 he should disclose you as a relative in the US. There's a high chance his visa will be denied though because of you living in the US.
As mentioned by @Mike E, if he has a chance of coming on visa waiver program, he should use it instead of applying for visit visa.