It's because he uses Tumblr. I repeatedly have this problem with my own and other tumblr blogs (the only pattern I have noticed is that they all use custom domains) and the only way to get around it is to refresh a few times and then it'll let you through. It's incredibly annoying, not sure if it's my ISP (who do you use?) or a problem with Tumblr.
Comcast Business.
I built my personal blog on Rails and use Heroku so hopefully people won't have an issue like that (Once I get around to actually posting something!)