First, I know you have already received some good suggestions here, but I would suggest that you start a new thread for this topic.

Next,the letter is a bit long and accusatory. I know this is all very frustrating (and we have all been there) but I would send a short note requesting a meeting (ie I would like to touch base to see how you think he is progressing and get some insight on how to plan for next year) Before you have the meeting, I recommend that you read this document. http://print.ditd.org/young_scholars/Guidebooks/Davidson_Guidebook_Advocating.pdf

I think it will help you frame your issues and think about other possible options for your DS. Good luck.