Hi
DS1 is 14 and has also just started to read adult books, having read most of the books already mentioned (Not Adrian Mole though, that's a great idea, must see if he'd like that).
At the moment he's reading books by Clive Cussler, cant remember what they're called, they're all action/adventure type novels.
Harlan Coben has started writing for young adults - his newest book Shelter is for that age group - so your DS might enjoy that.
Also, iBoy by Kevin Brooks. It's for young adults and it does cover some adult themes (a bit like the Muchamore books). DS really enjoyed that one.
John Connolly has also written for kids - 'The Gates' and 'Hells Bells' - these are very funny, and not anything like the dark Charlie Parker adult novels he writes. These are probably aged at a slightly younger audience (12-ish) but are still a great read (I read them and enjoyed them!).
Another book he read and enjoyed (it was one of the kindle christmas bargains) was Department 19 by Will Hill.
Oh, and the Patrick Ness trilogy (Monsters of Men, The Ask and the Answer and The Knife of Never Letting Go) were absolutely loved by DS1 and DS2 (just gone 13).
Hope you can get him a book he'll enjoy. It's great to see boys with a nose in a book
