One could argue that all fiction is speculative, but some of the books recommended herein are fiction, not "speculative fiction."
The terms is generally used to describe more subtle versions of sci-fi/fantasy.
The Leftovers by Tom Perrota is, I think, a good example.
It tells the story of a suburban NJ family after a large segment of the world's population has simply disappeared.
There are quite a few sub-genres of recognized genres.
In finding books your son might like try searching on Google for
Post-Apocalyptic Novels ( This is a growing sub-genre)
The Passage by Justin Cronin not only is post-apocalyptic but it involves vampires (victims of a bio-engineered virus that is). For what it is worth, the book received great reviews. Warning: It has graphic violence and is scary, but that may be just what your son craves.
There is a relatively new sub-sub genre: Steampunk. This usually involves Alternate History.
The Strange Affair of Spring-heeled Jack by Mark Hodder is a good example, although some will find it too much like a comic book.
A fine piece of speculative fiction with a silly sounding name is Mr. Shivers by Robert Jackson Bennett. Not written for kids, but a kid could enjoy it on its basic level, if they appreciate and can handle dark tales.
Finally, The Skin Map by Stephen Lawhead may be the best example that is most appropriate (despite the name) for a kid.
There are loads more if you are interested.