Depends how emotionally mature.
Agree Rosemary Sutcliff, all the Roman ones.
Hornblower, a bit dated but great. Go on to Maturin afterwards.
My side of the mountain
The good master, by Kate Seredy, and thd moving sequel about ww1 from the Hungarian side, the singing tree.
The Hobbit, then Lord if the Rings.
Again a bit dated, but the Roger Lancelyn Green retelling if the myths and saga of the norsemen and tales if ancient Rome, ancient Greece. A good grasp of myth will stand in good stead.
Gerald Durrell, birds beasts and relatives and my family and other animals. And all the others.
The longest journey.
Tarka the other.
Gavin Maxwell on his otter, can't recall the name of the book.
The silver sword
The dolphin crossing
A tale of two cities.
Silas Marner
The black tulip
The count of Monte Christo
Asimov short stories
The day of the triffids
The chrysalids
Agree holes. V funny.
Dahl's biography, boy
If he likes history, there are good abridged versions of Pepys'diaries.
Very stereo typed but coming back into fashion, Biggles. Not my thing....
Saki short stories