All of the Arthur Ransome books are a fantastic read, easy to understand and follow, and really quite exciting. One nice factual touch is that the boat Goblin that features in We Didn’t Mean To Go To Sea is a real boat and she still exists, now owned by The Nancy Blackett Trust. Ransome was living on her on the Broads when he wrote the book.
I would also suggest the Horatio Lyle books by Catherine Webb. They’re set in Victorian London and Horatio is a mad inventor/private detective, a sort of Steampunk Sherlock Holmes. He has two sidekicks, a rather posh boy, and a very smart, cheeky girl street urchin. Excellent books, I can’t recommend them highly enough. She’s also written four other books, the first two, Mirror Dreams and Mirror Wakes, are stunning, very much in the style of Roger Zelazney, but your lad might struggle with them a bit, but certainly worth a try. They’re a good adult read, too. I loved them. Catherine had the first one published when she was fourteen…