Mia,
I am more of the new comers and wasn't aware of your story but reading your post puts a big smile on my face. I am so happy for you and your family. It really does sound like you did the right thing for your son. And second guessing comes with the territory. Everyone second guesses, but given the progression you are seeing with your DS I don't think you need to focus too much on the what if games.
Also... I agree with everyone on the graph paper. If a child's writing is below par isn't it logical that they would be allowed to use the special pencils to help them control their writing? Graph paper is doing that for your son. It is simply a tool to help him feel comfortable and guide him to getting to the level he needs to be at. I think the teacher's response will be more in lines of why didn't I think of that?