He's eccentric to say the least. And his poor mom surviving on $74 a month! Too bad they can't put it in a trust or something for him, in case he changes his mind -- or at least give some of it to his mother so she can take care of him.
At any rate, it's not clear that he won't get the money. He just didn't show up to accept the award.
Ball told Nature before the medallists were announced that it was unclear what would happen — to the money or the honour — if someone refused the award.
And there's also the matter of the $1 million prize that's likely to be forthcoming from the Clay Mathematics Institute. I have a feeling the guardians of the math world are not unfamiliar with eccentric geeks and how to handle them, and will make sure the money gets to him somehow. But they may have to deliver a briefcase full of cash, or put the money in his mother's bank account, as I'm not at all sure this guy is sufficiently in touch with the concrete world to deal with things like banks.