I have seen students that appear to have reading fluency (rate and accuracy problems) and or comprehension problems make a "miraculous" recovery once on meeds. It is important to understand what was hampering your son's reading - was it an inability to concentrate/attend, difficulty with working memory? If so, it is reasonable that the medication made a positive difference. If he is an inaccurate and or slow reader due to weak decoding skills (understanding and applying the word patterns and sound symbol relationship), he might need more assistance beyond medication.
I have one child that is severely dyslexic (and has ADHD). While the medication helps him concentrate, it does not solve his reading problems. The other has relatively poor reading fluency - but this stems from an inability to stay focused. While she is not on medication, I have a hunch that medication would help her in this area.
I am glad that school is going better for him and that the teacher recognizes his improvement. I am sure all of you feel better about things!
