We are currently enjoying the Magic Tree House Research Guide #2 - Knights and Castles by Will & Mary Pope Osborne.
The research guides are a non-fiction set to go with the novels. He might enjoy this approach.
Some parts of history just ARE boring to some people (I personally would prefer to have a root canal - as long as I don't have to pay for it ; ) rather than study Early American history or the US Civil war). Switching to another topic for a while might increase interest. (Knights & Castles are FUN! : )
When one of the boys was young, we made a model of an actual castle (all the parts - bailey, barbican, etc) out of cake & candies. I suspect he STILL remembers that a castle isn't just the "house" part.
David Macaulay
http://en.wikipedia.org/wiki/David_Macaulay has great books with detailed charts/pictures/diagrams that might appeal especially to someone with Aspergers - you can just study the pictures for hours.