Are you active in your community's Arc or Down's Syndrome groups? What about the Council on Developmental Disabilities in your state? Here's the link for Washington's Council on Developmental Disabilities where you and your son can become more active advocates on a state level and meet others in your situation. There's also a book a friend of mine whose daughter has Down Syndrome suggested to me as well. Here's a link to a review of that book called, "I Just Am: A Story of Down Syndrome Awareness and Tolerance" by Bryan and Tom Lambke.
Also, try an exercise with you and your son together naming three things you are grateful for in your life. This is great, not only for Thanksgiving, but every day of the year, as we self advocates and our families get bogged down by the barriers that still exist in our lives.
I hope I was helpful to you.