I'd go for 'light' classics.
Funny reads: Douglas Adams, Terry Pratchet, Wodehouse
Swashbuckling adventures: Dumas, RLS, Tolkien
Crime: Agatha Christie (start with Roger Aykroyd), Sherlock Holmes, Dorothy L. Sayers
Classics: Wilkie Collins, Dracula, Frankenstein, To Kill a Mockingbird, probably some Dickens but I'm not a fan so can't comment, The Picture of Dorian Gray (DD1 loved this), Oranges are not the Only Fruit.
My teens are girls so not sure what contemporary stuff he might like, they like fantasy romance crap. If he liked Klara he'd probably like Never Let me Go. Maybe try Beryl Bainbridge, her novels are generally short, gorgeous writing but very bleak. Half of a Yellow Sun by Chimamanda Ngozi Adichie is fantastic about the Biafran war, Normal People by Sally Rooney might appeal at that age, Death and the Penguin by Andrey Kurkov. Tell him to just try lots of different writers so he finds what he likes.