Yes I second the Moomins, series, Tove Janssen - witty, warm, slightly surreal. Best read soon before he's too old for them? Noel Langley's the Land of Green Ginger is both funny and moving ; it's the adventures of Aladdin's son. My children loved Borrobil, magic unleashed on the eve of Beltane.
if you're also open to non-magic, Eric Kastner's Emil and the Detectives and The Parent Trap are both terrific, and fundamentally gentle - children of nine or so having to be resourceful and independent when they're caught up in adventures.
My son was gripped by Catherine Storr's Marianne Dreams, about children finding the drawings they do in the day becoming nightmares they must escape at night. Darker than the others above though (like the later Over Sea, Under Stone) - so he may not be ready for that just yet.
if he likes history, Legions of The Eagle, Henry Treece, is about a British chieftain's son captured by the Romans. Also darker in places (battles, slavery, betrayals) but riveting, surprising, and answers the question of what the Romans ever did for us