I would suggest looking into some of the Saturday kid's activities at local museums, art institutes or such. Many times the guided group activities can help form a friendship that can be continued in playdates. My experience has been that these programs pull a high percentage of above age/grade achievers. Additionally, Hugs to you and your son. It is really hard to experience the situation you descibe. Our family is still working through some of those issues.
actually, this is a good idea!
I am going to check right now if our local science museum has some.
