I haven’t read the Young Merlin books, but if he’s keen on Arthurian legends, you might want to try Gerald Morris’ Squire’s Tale series. They’re very funny and fit nicely into the established myths. He won’t find them much of a challenge, based on the other books he’s read, but they’re a lot of fun. Same theme but in a totally different style is Susan Cooper’s classic Dark is Rising series. They’re eerie books about the last great battle between the Light and the Dark, set in Wales and Cornwall (Dopers, am I right?) featuring a set of clever siblings and one Chosen boy.
Otherwise you could try John Boyne’s Boy in Striped Pyjamas. Buy it for him, and once he’s done, read it yourself. It’s set in WW2, about a boy, Bruno, whose father is in charge of Auschwitz. Bruno is only nine, and completely fails to understand the wider implications of everything he sees, which sets the stage for a tragic, but powerful ending.
I don’t know how popular they are in the States, but there’s an Australian author called John Marsden who’s written (among other things) a series of books called the Tomorrow series. The first book is Tomorrow When the War Began. Basically, small group of teenagers from a small rural town go tramping into a wild bit of bush for a week, and emerge to discover that Australia has been invaded by an unnamed enemy. DUN DUN DUUUNNN! The series is brilliant, with thumping action sequences, pitch-perfect characterisation and a fair bit of humour. For my money, the series declines in quality after the third book, but I was 11 when I read the first three and 21 when I finished the series, so your son may love the whole shebang. (Also, from the fourth book onwards the NZ army start saving the day, and while that’s neat, I can’t quite suspend belief so much)
Gerald Durrell’s Corfu trilogy is always worth reading. It’s a childhood memoir, about a family in the forties(I think. Or possibly late thirties) who relocate from damp and deary England to the Greek island of Corfu. The first book is called My Family and Other Animals and that, more or less, is what it’s about. He’s got about half a dozen other books, three about Corfu and the rest about his adult work as a conservationist and collector for zoos. He was a fascinating man who did a huge amount to change the role that zoos today have in the conservation and protection of rare and endangered animals.
I’ve always struggled with ‘classics’, but I devoured my parent’s Sherlock Holmes collection when I was about your son’s age. I also read the* Clan of the Cave Bear* series (but don’t give those to your son - they’re god-awful!), got really into the John Grisham books that were available - The Runaway Jury was a particular favourite, and I read *Dragonrider *books by the kilo. None of which re grreat literature, but I’m glad I got to enjoy them before I became a critical reader.
Okay, that’s enough for now.