Other people have already said The Hobbit by J. R. R. Tolkien, and I would agree. Otherwise, I don't know what else I would recommend in the high fantasy genre, or even in the fantasy genre more broadly. Well-written fantasy is hard to come by and well-written Christian fantasy is even harder to find.
I was a big fan of the Royal Diaries and Dear America books, although those are more geared towards girls, and I don't know if you're recommending these to a boy.
Kidnapped by Robert Louis Stevenson is an adventure novel set in 18th century Scotland.
I started reading the Horatio Hornblower book series long after middle school, but I think they'd still be appropriate for middle schoolers (or at least the first few books I read were, to my memory). The books follow the story of a young man in the British navy during the Napoleonic wars.
Island of the Blue Dolphins by Scott O'Dell is about a Native American girl who is accidentally abandoned on an island, and must learn to survive on her own. Sort of a sad book, but still enjoyable.
Dancing Shoes and Ballet Shoes by Noel Streatfield both focus on sets of adoptive sisters at dancing and theatre schools in England during the 1930s and 1940s. Due to the subject matter, I'm not sure how much boys would enjoy them, but I would recommend them to middle school girls.
I read the Sophie books by Nancy Rue when I was in 4th and 5th grade, but I still think middle school girls would like them. They deal with a lot of Christian themes, as well as other topics relevant for girls in that age bracket, such as friendship, bullying, and getting along with one's parents. Nancy Rue wrote other book series as well, but the Sophie series was the one I was most fond of.
I also HIGHLY recommend reading books beforehand, or even at the same time, as your kids. It gives you the opportunity to discuss the book's themes and content with them, which can be really good if you're worried about a book having murky aspects to it. That way, you can talk through them with your kids. I'd also see if you could find a book club for Christian middle-schoolers in your area. I joined a book club for Christian homeschoolers early in high school, and not only did I make lots of friends, but I read wide variety of books.
I'll come back if I think of any more books.