LibertyD--
The Color of Dark, has a love interest as well as being all about grownups. On the other hand,
The Curious Incident of the Dog in the Night-Time is about a ten year old kid who has much more math ability and many fewer social skills than your nephew.
Reading
The Curious Incident... with your nephew might be a useful form of self-exploration, defining individual differences and getting into "real" literature as opposed to the Wholesome Childish Books. Or it might not.
When my football mad son was about 11 he picked up one of my library books,
Alphabet Jackson about a football player and read it cover to cover. Then he asked me what I thought of the book. I said I thought there was too much football for my taste. He replied that there was too much sex & drugs for his liking--and not nearly enough football.
You might check out David Eddings:
http://scifan.com/writers/ee/EddingsDavid.asp
or some of the early Heinlein:
http://scifan.com/writers/hh/HeinleinRobert.asp
Better yet, make one of your errands a trip to the local library. First, get him a library card of his Very Own (and the consequent responsibilities of taking care of books and keeping back of the due back dates). Second help him confer with the Children's Librarian about what he might like to read.
If you're worried about his social behavior, call ahead and smooth the way. You are a home schooling aunt with an AS kid who needs social skills as much as books right now. For all I know there are books out there for 5th and 6th graders dealing with boyish ineptitude and happy endings.
Hold your dominion.
P.S. Make the library a regular, weekly event. Structure is important. Besides, you can pick up some relaxing reading of your own to set a good example!