In the HP vein, the first thing that comes to mind is Diana Duane’s So You Want To Be a Wizard and its sequels. This is one of those series that starts off pretty strong, but toward the end, it’s like the same plot written over and over. I wouldn’t run out and buy all of them at once, but the first two at least are very entertaining.
Daniel Pinkwater’s books are classics (IMHO at least), especially Lizard Music and Alan Mendelsohn, the Boy From Mars. They are not really magic or fantasy, more like a very humorous non-reality. There is something about Ron Weasley that reminds me of a Daniel Pinkwater character, so I have recommended these with success to young HP fans.
The House with the Clock in Its Walls series by John Bellairs is good, and one thing I like about it is that the books switch back and forth between male and female main characters. Also, illustrated by Edward Gorey BONUS! I have heard some children find the magic/supernatural elements in these a little too scary, but I’m sure it depends on the individual child.
For a girl, Sorcery and Cecilia or The Enchanted Chocolate Pot, by Patricia C. Wrede and Caroline Stevermer, cracked me right up. It has a sequel that I haven’t read yet.
Sadly, The Secret World of Og by Pierre Berton seems to be forever going in and out of print, I think it’s in the out cycle right now, but any decent library should be able to get it. You’ll be happy to know this is a CANADIAN MASTERPIECE of kidlit. Five children discover a secret world inhabited by weird creatures under their house.
A dark horse option – the wistful and sometimes downright eerie Green Knowe Chronicles by Lucy Boston. These are perhaps a little slower-moving than your son and his friend are probably used to. They’ve got kind of a weird gothic vibe going on, a family manor inhabited by the ghosts of children who used to live there back in Ye Historical Times. Very, very English.
Darn it, I can’t remember the name of this book, but I’ll describe it here in case another Doper knows the name. An American family moves into an old haunted castle (possibly the father is on sabbatical in England or something?) and encounters the castle ghost. With sort of a “Ransom of Red Chief” twist, the American kids are obnoxious and not at all afraid of the ghost, but eventually befriend him. It’s a hoot.