If he's into horror, why not go back to the classics - Edgar Allen Poe. The fact that Poe wrote short stories might help, as well. Also H. P. Lovecraft. And some Ray Bradbury (especially good as he often wrote short-short stories). If he's open to adventure tales, Jack London is good. Mark Twain for humor.
Also, read with him, read aloud to him, and talk about what you are reading.
Also, find books related to his favorite TV shows or other interests. For example, he might both watch The Dead Zone on TV and read the Stephen King original novel.
If he's open to science fiction, the classic Heinlein juvenile books are good.
If he's open to fantasy, I think the Young Wizard series by Diane Duane is great, too.
At his age, I was a big Hardy Boys fan, but that doesn't seem up his alley.