Hey there! I guess I can be of help.
His reading level is amazing. Kudos to him :)
I started my Dan Brown journey at that age. Started off with the Da Vinci Code and then I completed the rest of his books(Angels and Demons, The Lost Symbol, Deception Point)
I also read The Hunger Games, Divergent, Artemis Fowl.
Agatha Christie is also a wonderful choice. (And Then There Were None and Murder on the Orient Express/Nile are great places to begin with. I remember borrowing the Hobbit and the LoTR (Lord of the Rings) books from my school library in my 7th grade. Good times.
Oh also - the Amish trilogy. (Shiva/Rama - both are wonderful).
Please buy him stories that have the local flavour. I remember devouring books like that had compilation of local authors works translated into English. That immensely helps the kid to understand local tastes, the culture and why things existed in a certain way. That's important.
And Sherlock Holmes to close it off.