[zahllos]

I can believe he wasn't sure mathematics was his calling during high school. I wasn't sure I would do a degree in mathematics until near the end, then I did (am no way near a fields medalist). But this is time up to 18-19. There's a lot of time after that.

However he took a course high in mathematical content and it sounds like he must have switched to mathematics for grad school, I.e. after his undergraduate or masters. In other words, he did the preparation. If it is anything like physics courses I have seen they feature a lot of mathematics courses anyway.

Some people have a lot of natural ability in mathematics but this is not enough to do good research. There's no way around hard work.

Also, dropping out of high school in many places significantly reduces your chances of going to university because you do the qualifications you need at the high school equivalent.