My DD12 has autism, and she is the type of obsessive reader you describe. But I am not on the spectrum, and I was too. I still sometimes read while brushing my teeth and riding in the car (although not driving!). When I was about four, I read part of a medical textbook belonging to the mother of one of my friends, and told her it was "silly". I wouldn't worry about it unless you feel like it is interfering with other parts of his life.