My advice would be to turn him loose on
Project Gutenberg and just let him read whatever he wants. The
bookshelf category is probably a good starting point. He can pick a category that looks interesting and go from there. He'll figure out what his interests are on his own. I tend to agree with Khross that
what he reads matters far less than
that he reads. To that end, giving him free choice is one less hurdle (and a big one, at that) to put in his way.
Similarly, he'll probably seek out an appropriate difficulty level on his own. A funny thing about human nature is that we like to get better at things, and we tend to get bored with things that we're already good at. For instance, your son probably spends less time playing video games that he finds easy than he does playing games that provide some challenge. Reading is really no different. He may choose some easy reading now and again (particularly at first), but he'll likely discover pretty quickly that this gets boring. In order to seek out books that engage him with more complex themes/plots/etc., he'll have to tackle more complex writing, vocabulary, and so on.
Project Gutenberg could be a really good resource because, first of all, it costs you nothing above what you're already presumably paying for internet access. Secondly, pretty much anything he picks there is going to be "worthwhile" reading by any standard. As I said, I don't think this matters (at his age) nearly as much as people think, but nevertheless, there you go. Best of all, most (all?) of their material these days is available in various e-reader formats if he has a Kindle/iPad/whatever to package it all up into an easier reading format with (at least speaking for my Kindle) easy access to a dictionary. And on that note, make sure he always has a dictionary available to him however you go about this.
On a more IMHO note, I would actually make it a requirement that he makes a good effort to use the dictionary/encyclopedia/etc. before turning to you for help with a subject or word that he doesn't understand. I can expound on my reasons for that if you'd like, but in a nutshell, this was probably the greatest thing my parents ever did for me. And while it may have pissed me off a bit at first, I came to appreciate it pretty quickly. Again, IMHO, but reading skills are ultimately a means to an ends. If he doesn't value the ends, of course he's always going to resist the means.
Edit:
Also, a long-term recommendation:
the five foot shelf.
Available via Project Gutenberg (albeit with some alternate editions)
A fair amount of that list is probably not going to appeal to a 10-year-old, no matter how smart they are, but a lot of it probably would (ex. Don Quixote, A Thousand and One Nights, etc.)
Edit 2:
Heh. Dr. Eliot wasn't kidding. I don't have an exact page count, but based on ~425 pages per volume, if you read 15 pages a day, it would take you just under 4 years to read the five foot shelf. One of these days, I really need to commit to reading it myself.