Caleb Carr wrote some historical fiction, including The Alienist and The Angel of Darkness. They’re set in Victorian New York. Your son might enjoy them.
I used to love to recommend Preston/Childs books to teenage boys when I worked at Barnes & Noble. I even had a couple of kids come back, thank me and ask me for more recommendations. I’d usually start them on Riptide, or The Ice Limit. They’re both fast paced, exciting, read 'em in a couple of sittings kind of books.
Another one, if he liked The Da Vinci Code, is The Genesis Code, by John Case. I think Dan Brown must’ve read this book – it predates The Da Vinci Code and has a very similar plot. It also has much more interesting characters and John Case (a pseudonym for a husband and wife writing team) can actually write.
I assume you mean Angels and Demons? Brown has also written two other books, not in that series, which in my experience tend to be liked by people who liked The Da Vinci Code and hated/looked down upon by people who hated/looked down upon The Da Vinci Code.
You might find some good ideas in this thread:
Recommend Books for 16-year-old boy
He might also like The Amazing Adventures of Kavalier & Clay by Michael Chabon.
OP: My overweight son went on a half mile walk yesterday, and really enjoyed it! Can anyone suggest some other, easy exercises my unathletic son can try that he might enjoy?
Frylock: How about the Boston Marathon?
My daughter recently read Jonathan Stroud’s Bartimaeus Trilogy. It’s also about young a young magician, but the magicians aren’t all good guys and the “muggles” aren’t all bad/boring. It’s set in a fictional London where the magicians have all the power and regular people are treated as second-class citizens, and there’s an element of mystery involved regarding who is betraying whom. I thought it was very well done and also very funny in a lot of places. The title character is a very sarcastic genie.
What about other Dan Brown books? There’s more than two out. He’s got four out, actually. I assume the other one he read was Angels and Demons? If not, that one for sure since it’s a prequel of The DaVinci Code. If so, then give Deception Point or, my personal favorite, Digital Fortress a try. All are by Dan Brown.
But really, as I remember it, the book can be enjoyed on different levels, and it just might push someone up a level or two.
But you’re probably right to express caution. 
Count me as one who does NOT recommend Foucault’s Pendulum. The thing with the Illuminatus! Trilogy is that, silly and incoherent as it is, most of the conspiracies are based on real (theorized) conspiracies and that gives it an internal inconsistency lacking in both Foucault’s Pendulum and Angels and Demons, which are too linear to be describing Bizarro World. Shea and Wilson researched their books heavily while Eco and Brown use the Illuminati as a framework on which to hang stories that didn’t really need the Illuminati. To them it was just a cool word but Shea and Wilson ended up half-believing in it.
In a similar vein - historically if not thematically - you might try “The Historian” by Elizabeth Kostova. It does have plot elements including the Templars and the Cathers - and throws in Dracula as an added bonus. I think it just came out in paperback.
Other than that, echo support of “Foucault’s Pendulum“ and “The Eight.” Also, he might like “The Name of the Rose” (my 18 yo cousin did, and then he went on to “Foucault’s Pendulum).
Thank you everyone. Tons of great suggestions - I head off to the book store tomorrow.
As for the $1000 gaming computer - That is so over my “you’ve got to be kidding, this is so far out of my price range I can’t even consider it” type price -that anything over a grand might as well be a million. I did find a web site that sold re-furbished computers that had all the right graphic cards for $1,100 FWIW.
I may get castigated for this, and I want to state for the record that it is not representative of my current taste in fantasy, but I and many kids your son’s age, or a bit younger, got hooked on fantasy thanks to “The Belgariad.”
You can get both volumes of the collected series for less than 50 bucks, and it takes either three readings of the Belgariad or one reading each of the Belgariad and the Malloreon (which I think is now also available in collected paperbacks) to realize that Eddings just runs the same stock characters through the same stock maze.
But before I realized that, they were a thrilling read. The characters are simple, yes, but simple = identifiable, and the holes in continuity are not so great that they’re easily picked up the first time through.
I highly recommend them for a kid that’s starting to get into reading.
Oh gosh yes, Follett. Except for the one about the twins, forgot the title, silly story and too many improbabilities.
I like Ellen Cherry’s suggestion of Gary Jennings – real page turners, but there’s some extreme violence in Aztec.
cher3, I’m reading The Bartimaeus Trilogy now – I’d have loved it even more at 15.
Maybe he’d like The Golden Compass by Philip Pullman, especially with the movie coming out soon.
You yourself are a fictional character from a novel which has a bit of the history/conspiracy thing going, sort of, aren’t you?
Maybe not for british school boys, though. 
-FrL-
Yes, but most books aren’t so obviously written while having sex and taking drugs.
If he might like books in a Da Vinci-esque vein, have a look at Stel Pavlou’s Decipher which is the same kind of style but it’s about Atlantis.
That’s kind of what I was thinking! Foucault’s Pendulum is a really great book, but it’s the kind of book that could scare off someone who isn’t a really enthusiastic or strong reader. One of the reasons The DaVinci Code was so popular is because it’s incredibly easy to read.
I thought the suggestion of Ken Follet was a good one. Steve Berry also writes historical conspiracy thrillers that are pretty good.
Right you are. Get some Tom Robbins and read up! Skinny Legs and All, Jitterbug Perfume, Another Roadside Attraction. Good stuff!
Early Cussler is undemanding adventure reading and is OK - I stopped a few years ago when the last Dirk Pitt novel I read was such a nasty piece of writing I decided never to read Cussler again.
YMMV
Si
Damn you to hell! I wanted to recommend Preston/Child, and had forgotten about Case - Genesis Code and (If I recall the title) The Fifth Horseman were decent…
I like all of the Preston / Child books. I started with The Relic, and NONE have disappointed. A 16-year-old who likes a cool story will devour them.
Joe