Apparently it's really common for kids with autism to have a particular subject that they become experts about - or obsessed with.
For some it's trains, or geography, or calendars. We know a boy at school who is all about vacuum cleaners. It's all he talks about.
With an autistic child one of the best ways to get them interested in learning or working on something is by picking up on their favorite topic and really expanding on it. My son's therapists are great at this. Want to get him to work on drawing a line in a windy maze? Make it about airplanes - you have to get to the end to get the airplane to the runway. Want to get him to read? Give him a book about airplanes. Want to work on fine motor and building? Build a lego airplane.
So I am feeling like we scored the jackpot with our son's current obsession with airplanes. At least it's a topic that the rest of us find interesting too. We have a membership to our local flight museum, and it has become one of our family's favorite outings.