Because so far he's staying true to the motivations as he described them; he's using business to help solve big problems - as opposed to other entrepreneurs, who use big problems to help their businesses. He's done nothing so far to make me personally doubt that his motivations are as noble as he says they are.

I'm not saying that he won't eventually turn into yet another selfish business baron. That would be a very sad day, though. We need more people with noble intentions and means to execute on them - and such people need to be encouraged, not constantly doubted.

To me he seems like a genuinely noble guy, especially among many other successful capitalists.

wouldn't say noble, just consistent and different from your usual businessmen who treat amassed capital and/or net worth as a measure of success.