I think he did a combination of dumpster diving and bartering his labour for things. I haven't actually read the book, but I'd be surprised if gardening didn't play a huge part in it.

He's also in England, so the government would give him a lot of things an American wouldn't get -- the most obvious being health care.

He also has a blog: http://www.justfortheloveofit.org/blog
I find it a bit insufferable, if interesting.