I think at 15, the most important thing is that he is steered in the direction of books he will enjoy.
Terry Pratchett for instance, is not great literature, but very enjoyable, has a lot to say about folk lore, perceptions of the world etc, lots and lots of oblique references that you reread to try and get - and then go off and read those books. Nothing objectionable in them either.
Garth Nix, Phillip Pullman also very suitable for that age group. In this case Phillip Pullman is beautifully written, and a lot to think about.
I've heard that the Marjory Blackman Noughts and Crosses books are very good, but don't know about appeal to 15 yr old boy.
If he fancies some very accessible history, the Forgotten Voices books are very readable first hand accounts of 1st/2nd world war events and might suit.
But tbh, at that age I just wandered round the library and chose books I thought I might like - probably by far the best thing for him rather than being presented with a reading list by his parents