@coodawoodashooda The first one that was more 'adult' in nature that he couldn't put down was the CHERUB series, by Robert Muchamore. It's about deniable child spies, and over the series includes swearing, underage drinking, sex, violence, teen relationships, drugs gangs, and so on, which gets a bit more involved in the later books in the series (when the characters are older than in the earlier books).
My boy's a pretty happy reader anyway (has previously gone through series including Harry Potter, Skullduggery Pleasant, Percy Jackson and other Rick Riordan series, and currently waiting for the fourth Nevermoor book to appear (Nevermoor books are great for family road trips as an audiobook - enjoyable for adults whilst being accessible for younger siblings, too), but Robert Muchamore's books are many degrees closer to plausibility and the real world, so are a bit more pointed than books where the violence involves witty, fire-throwing skeletons and lacks the (relatively tame) explicit content.
Hendersons' Boys and the Robin Hood series, also by Robert Muchamore, have been devoured equally quickly. He's got things like the Special Forces Cadet Series by Chris Ryan and Christopher Paolini's 'The Inheritance Cycle' on his wishlists.
We've been lucky that choosing books and reading is intertwined with growing independence for our children. It's not something we planned but, as the local library was one of the first places we allowed them to venture to independently, and they have online library accounts which they use to reserve books and check when they're available to collect, and they take themselves to the library after school to collect and/or return books, they have control and autonomy over their reading. I check what they've borrowed and look the books up, and if they might be a bit mature / over their heads, I let them know that the 'wisdom' of the internet suggests its more suited to older readers and that I'm available if they've got any questions.