Hi! My 10 year old son is obsessed with the audibles of all the Rick Riordahn books, he has read/listened to all the Percy Jackson, Kane Chronicles, Lost Heroes, and 39 Clue Books..... I think he would love Harry Potter but they are not on audible.com.... what else would you suggest?
try books such as the heir series, the first one is the warrior heir. also, the hunger games trilogy is, i believe, out on audible. they are for a bit older children. also, depending on his reading level, check out series like, The Land of Eyelon, and the Sisters Grimm. they are all very good sci fi/fantasy reads. good luck! remember, if he loves to read, he will read anything!