I actually think you're doing an awesome job with what you're doing. I would encourage you to do what you're doing and exposing him to different types of reading materials and subjects. There's a catch when you start to formalize or provide more structure to what you're doing - you may lose your ds's motivation, interest, and attention.
My ds9 has dyspraxia too and possibly dysgraphia who has been un/homeschooled for the last three years. I have loosely followed a general curriculum series for the elementary years just to cover the basics or gaping holes. Others here my chime in with suggestions, but really your investment in your son and his education is what's key and what will pay off here, imo.
