I would not make your son follow the audio books with reading the physical book. It sounds like the reading for enjoyment arrived with the audio books which should be encouraged. This can lead to reading physical books with the right text format later. My son started reading lots of graphic novels with larger text fonts, and now reads regular books voraciously. He has jerky eye tracking as well as CAPD and mild dyslexia.
I still get the audio book or the movie for English or other projects as a supplement to the reading. Getting the information multiple ways seems to help my son process and synthesize it well. He uses Youtube and other online sources for research for projects too, which is still ok in high school.
