Of course, top of the list is C. S. Lewis’s Narnia series.
The Spirit Flyer Series, by John Bibee (There are 4 more books, but these are the ones we read aloud together when our boys were younger)
The Archives of Anthropos series, by John White
The 100 Cupboards Trilogy, by N.D. Wilson
The Dragon King Trilogy, by Stephen Lawhead
I haven’t read all the books written by Stephen Lawhead, but I’ve liked every one I’ve read, and most of them would be good choices for boys as well.
Chiveis Trilogy, by Bryan M. Litfin. You might want to check out my review of audiobook edition of The Sword.
The Pendragon Cycle, by Stephen Lawhead