I wasn't familiar with this series, but I looked it up and the author, Bruce Coville
is well known and has several series. I'd start with some of his other books. I think DS enjoyed the Sixth Grade Alien books, but I realize those aren't quite right for you because of the school setting. BTW, although it sounds like your DS likes the fantasy stuff, if you want to try something else at some point, Coville's Shakespeare retellings are pretty good.
Another author that DS liked was Richard Scrimger. The Nose From Jupiter
marries fantasy and humour very nicely. There are sequels too. It's been a long time since I read them and I think there is some school stuff, so they might not be perfect for you.
You could try the Pure Dead Wicked
series by Deb Gliori
. They are a little more advanced than the level you are looking for, but they have that same blend of magic and humour and gore that kids love, so perhaps that will keep him interested as a read-aloud. It's funny, the Lexile
measure for Pure Dead Wicked
is higher (1020L) than The Lion, The Witch and the Wardrobe
(940L) but I would have pegged Pure Dead Wicked
as much lower.
Also at a higher level, but I'll mention them because DS loved them at that age:
series by Brian Jacques
The Edge Chronicles
by Paul Stewart and illustrated by Chris Riddell
If you don't get other suggestions or can't find anything else, it might be worth trying them. I found that if the stories were really good, my kids could listen to a fairly high level of writing. Another thing you can try are illustrated books of myths and legends - Greek, Roman, Norse, Asian, Aboriginal etc.