A few to try:
Patrick Ness's Chaos Walking trilogy: The Knife of Never Letting Go, The Ask and the Answer and Monsters of Men, also his A Monster Calls.
Code Name Verity by Elizabeth Wein, which is popular with girls but also works for boys.
If he's into magic, the Old Kingdom series by Garth Nix - Sabriel, Lirael and Abhorsen, to start, are brilliant. The Graveyard Book by Neil Gaiman is also excellent, spooky and weird, and Gaiman's written some other good books (not American Gods for a 13 year old, though - maybe Neverwhere instead). If he likes Gaiman try him with Terry Pratchett - Guards! Guards! would be a good place to start.
My Sister Lives on the Mantelpiece by Annabel Pitcher, really really sad in places but great.
Bog Child by Siobhan Dowd, about an Irish family during the Troubles.
Some people don't like Michael Morpurgo but I thought his Private Peaceful was excellent.
I wonder if he'd like audiobooks? They count (to me) as reading, as long as he doesn't zone out so they become background noise.
If he's looking for books in a specific genre he could try Accelerated Reader's Bookfinder. You don't have to be using AR to use it. Select a choice from the first page (it doesn't matter which, they all give the same access) then click Advanced Search on the next page. Interest Level will be Middle Years +, and set ZPD range to something like 3.5 - 10. That should cover modern books for a bright 13 year old. Then he can choose a topic from Additional Criteria. The ratings are from pupils using AR so he can see what other kids thought of the books.