M.R.
M.,
I highly recommend that you consult a professional with his educational evaluation data in hand. Intervention is provided based on his needs, not his diagnosis. You are stabbing in the dark with such broad categories as reading and writing and there are very specific interventions that will be helpful, and others will not help at all.
He should have an IQ test that will show you how he will process information best, and academic testing that will identify what issues are problematic for reading and written expression. Most children with ADHD will have peaks and vallies in the testing data that is going to differ from kid to kid.
Once you have this data, you can get information on interpreting it from WWW.wrightslaw.com in an article called " Understanding Tests and Measurements for Parents and Advocates" Then you can find strategies that will be helpful from Dr Mel Levine or Dr Russell Barkley's books.
M.