I thought there'd be about as many answers as there are people, but not so many answers here :p The limiting factor to me is I think for a 6 year old, it would need to be kept simple, which might rule out most people I think of as real life superheroes ;) And with that in mind, I'd just go with someone or barring that, something that interests him personally.
When I think of the literal concept though, Stanislav Petrov "the man who single-handedly saved the world from nuclear war" and Nikola Tesla, who successfully put magic into "any sufficiently advanced technology is indistinguishable from magic". Those, however, might be bit beyond the age category. How about J.K Rowling, is that a thing? There are many great authors, but her personal story is among the most inspiring.
