Oh B., I feel your pain.
Most poeople have no idea how hard you have worked to have him just barely fit in, and then to be judged based on the progress of the typical kids around him. I have so been there with a play group.
I have some very direct advice for you. These children are probably not his friends. It took the children playing with my child's imaginary friend, but not letting her play for me to really see that it was not doing her any good at all to be with these children. She was not as happy as I wanted her to be there. This was my oldest child. I was in this group as much for me, as I was for her, and it hurt like fire to have to leave it, but we did.
We did have a better experience with our younger daughter in public school. She had some excellent teachers who guided the young people so that they did not catch on to my daughters differences until they got a little older, but they eventually out grew her, and we had to seek out more appropriate social settings.
I won't lie to you, it is a very loney road for these children, and it will be heart break for you many times over. We have recently found a wonderful school in our area that has a half and half mix of peers to developmentally disabled children. What we found is, our girls do so much better in this enviornment, they have friends they can relate to, they go to parties, the phone rings. They do have a peer group, and your son will find them one day, so there is a light at the end of the tunnel. My oldest has asperger, and she can sniff out the one other aspie in a croud of 200 in just a few minutes. Your son will do the same. Find him some peers who will accept him as he is. You will also find that he has more acceptance from very gifted children without disablities. They seem to have less need to make fun of our children, but they may also have less and less in comon with them as they age, if your son has a cognative impairment like our youngest does.
I guess what I am saying is, yes, he will lose some relationships because of his disorder, but these are not his friends. You will feel the sting as much, or more than he does. Do not give up, find some peers who he can relate to, and sometimes, one or two friends is more than enough. He may do better one on one anyway.
If he is not yet in a social skills class, check with your speech therapist, or ask your child's psychiatrist, these can be great resources, both for learning, and making friends. My oldest has two friends from a social skills class she took 5 years ago, and they still get togehter now and then to do things.
You have a community. We are everywhere, and we are looking for you too!
I am so sorry that this has happened to you, and I feel your pain, because I have felt it before too.
God bless,
M.