As a teacher, I have seen IEPs which require that students be given access to a computer for writing. I allow some students to go to their special ed room to work on a computer when needed. As an adult with ADD, I can remember the reall and profound impact that the introduction ofthe PC had on my ability to write. I remember the terror of facing a blank page in the typewriter, and need to cut and paste - literally - papers to form a coherent whole. With personal computers writing has become a joy. Well, perhaps not a joy

but something which I enjoy doing - it's still work. It still requires revision. But that too, is part of the process of learning how to write.
Which brings me to my second point - I'm pretty ambivalent about "speech-to-word" software. I understand the challenges your son has, but I do think that it's important that he master the basic mechanics of writing. He won't have access to such software in most everyday situations. Writing requires hard work - even under the best of circumstances. Yet, there is something about the printed word - reading it, thinking about it, manipulating it into final form - that is irreplaceable. I believe it is a skill that will serve hime well in writing as well as in math (don't get me started on kids reliance on calculators!)
Good luck - be a powerful advocate for your son - if he needs to use a computer to manipulate his words, makje sure that he has access to one. I can only tell you that as an adult who has really "been there" - there really is hope!