They're not mythology-based, but your son might enjoy the Tripods series by John Christopher. They're not very scary. DS6 has in the last six months enjoyed some books by William Sleator which are not too scary either, more like suspenseful young adult science fiction; they don't tend to run in series, though. The Hitchhiker's Guide books are probably in his reading comfort level, and aren't scary. The Ringworld series, ditto-- more silly than scary. The Piers Anthony Xanth novels aren't scary in my opinion, and they have some mythological elements and references woven through them, but you might have to vet each one for adult scenarios if you tend to keep adult romance from him (nothing graphic in those, though).
IIRC Rick Riordan has some other series too (I just looked them up: "The Kane Chronicles", based on Egyptian mythology, and "The Heroes of Olympus", the first book of which DS brought home and read a while back).
