Book recommendations for 13 year old boy

Some fabulous suggestions here. I will try and remember what mine enjoyed at that age but in the meantime I just wanted to say- don't despair! My DS went through a bit of a dry patch around that age but he got back into reading again and hasn't stopped since.
I do remember him liking Holes by Louis Sacher, Lord of the Flies, Animal Farm, To Kill a Mockingbird. He loved Tolkein but I am struggling to think of any other fantasy titles.

Pretty much anything here:

https://www.foliosociety.com/uk/fiction

Three more and then I’ll go away … Interconnected:

Thomas Mallory’s Le Morte d’Arthur (think I read it at around that age).

T. H. White - The Once and Future King (just everything)

and his Goshawk - not about King Arthur!

@PersisFord - I think we may be twins! So with you on that episode in the Rosemary Sutcliff novel; I was traumatised for about a year after re-reading it in my fifties. And I was poised to say that if you have a bookish nephew the best thing (aside from sharing books) is taking him, as often as you can, to the theatre. (Some of the most blissful moments of my life have been late night post-theatre chats whilst walking back to our hotel with my bookish nephew. Took him to countless things in his city, but it was always particularly fun, when he was young, to introduce him to other cities with a play at the centre of the trip.) I don’t mean children’s stuff; grown up plays, whether freshly written or Shakespeare. Wonderfully, he sometimes noticed things on stage that I hadn’t.)

You don’t mention Susan Cooper, but he may well have read all the Dark is Rising novels already. Another of hers Ghost Hawk is well worth anyone’s time.

If he hasn’t read things like

Erich Kästner’s Emil and the Detectives

or

E. L. Konigsburg’s From the Mixed Up Files of Mrs Basil E. Frankweiler

now might be a good time, before he feels too grown up for them.

Entirely different - the entire oeuvre of Josephine Tey is absolutely unmissable. You wouldn’t think a tween boy would spend a weekend utterly fascinated by

The Franchise Affair

but I’ve seen it with my own eyes! There’s a smidgeon of ‘romance’ in most, but it doesn't get in the way of seriously masterly writing and storytelling.

Charlie Higson The Enemy series

Garth Nix if he likes fantasy? Hatchet or Going solo could be good for his age also?

Lots of the above! Don’t think you have The Loop trilogy yet by Ben Oliver. My son loved them at this age.

Another vote for Hitchhiker's Guide! The Sherlock Holmes stories are also classics without being too challenging in terms of length or overly archaic language.

He may also like China Miéville's books - Railsea is specifically written for young adults but he could try Perdido Street Station if he likes fantasy and sci-fi or The City and The City or Kraken if he prefers something more literary.

I second everything @grannyjacob says. (Though can’t vouch for Morpurgo as I’ve never read him.) The rest of her list was my childhood / teen reading.

My first suggestion might be controversial …

John Buchan’s Richard Hannay novels beginning with The 39 Steps.

They’re full of thrill and adventure, and highly absorbing. (And I think each succeeding novel is better and better.) But they do contain some words and attitudes that are very much ‘of their time’. So you may need to be ready with clear responses if he’s surprised. I’ve loved them for decades, own the Penguin volume but made a present of the Folio Society volumes to someone of your son’s age.

Second suggestion:

Mervyn Peake’s Gormenghast trilogy. Entirely suitable in every way, tremendous storytelling, wonderful language. It’s described as fantasy, but it isn’t strange creatures going off on a quest. He might love it. Or hate it.

Was planning to come back in the morning, but don’t want to forget

Douglas Adams’ The Hitchhiker’s Guide to the Galaxy

if he hasn’t read it?

My son isn’t much older than yours. He really likes the works of Alan Gratz

Have a look at Robert Muchamore's Robin Hood series.

Gone series by Michael Grant. Son loved them, also a big Hunger Games fan.

He enjoyed Eragon (series of 4) too

Impossible Creatures?

Abi Elphinstone books?

Philip Reeve - Mortal engines etc
Raymond Feist - start with Magician
not adventure but I alway suggest Randall Munroe's What if/How to books for tricky teenagers.

I would also consider some graphic novels (unless he doesn't like them!)
Wynd
Aquanaut
Goblin (Grissom)

My two early teenaged DSes really enjoyed ‘The Scythe’ by Neal Shusterman.

One of them has also just finally finished the first Lord of the Rings books. Although it took him quite some time. He had previously read The Hobbit.

I also have a bookish nephew and it’s so tricky! These have been successful recently:

MG Leonard books.
Ted Hughes - Ironman
Stig of the Dump
All the Anthony Horowitz books
Gillian Cross- After Tomorrow
Jimmy Coates Killer (can’t remember author)
The Terrible Thing That Happened to Barnaby Brocket - John Boyne

The Cherub series is good, goes done well with kids you like Gone & Mazerunner.

Charlie Higson Young Bond series?

Has he read anything by Alan Garner, Rosemary Sutcliff or Michael Morpurgo? The three of them have books suitable for a varied age range. When I was his age (many years ago, I’m in my 70s now), I loved John Wyndham, Arthur C. Clarke, H.G.Wells.

My son at that age enjoyed a book series by Michael Scott starting with 'The Alchemyst: The Secrets of the Immortal Nicholas Flamel' - fantasy series with mix of historical figures and mythology.