I take it that you don't know this child well and aren't familiar with what he's reading? It's a gift, perhaps? That's tough because you could be buying him something he already owns. A gift card to a bookstore may be a better idea. In case a gift card doesn't work, here are some suggestions:
Books by Roald Dahl (eg. Charlie and the Chocolate Factory, Danny the Champion of the World, the BFG)
Judy Blume (eg Tales of a Fourth Grade Nothing, Superfudge)
Edward Eager (eg Half Magic)
The Diary of a Wimpy Kid is popular with that age group.
Bunnicula (series) by James Howe
Redwall (series) by Brian Jacques (a long series which may keep him reading for quite a while. There is an animated television adaptation. It's an adventure series with animals in a kind of medieval setting - lots of battles and feasts). It might be a challenge, depending on his reading level.
If he's reading Harry Potter, he could manage the Percy Jackson series by Rick Riordan (who also has other series you could check out).
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