1. He went out of his way to obtain that information because there's no point being a whistle blower without having some evidence to back up your claims.
2. He's a college drop out, not a high school drop out (or at least that how it reads to me - a non-US citizen. I may have misunderstood how your education system works). Also, his family also works for government agencies, so that probably added weight to his application.
3. Because of the secrecy of his job and the clearance he had. It doesn't sound to me like he was your typical sysadmin.
4. He actually addressed that one himself: On May 20, he boarded a flight to Hong Kong, where he has remained ever since. He chose the city because "they have a spirited commitment to free speech and the right of political dissent", and because he believed that it was one of the few places in the world that both could and would resist the dictates of the US government.
I can actually sympathize with your skepticism there. Some bits of his story does sound quite hard to swallow. But on the whole I'd say his story seems more plausible than made up. Personally I find it less likely that the US government aren't recording and monitoring that amount of traffic, even if the whistle blower does turn out to be a hoax. And I'm sure many other "democratic" counties are doing the same as well. We've seen how the content industry can basically buy police time and have servers taken unlawfully (in the case of Kim Dotcom) - so I'd be astonished if the government themselves didn't have even further reaching powers.
edit: I really wish you hadn't deleted your comment because while some of the questions had already been answered in the Guardians report, you did raise some worthwhile points. I just hope it wasn't kneejerk down-voters that made you choose to delete your comment (as such voting -in my opinion- hinders discussion)