Sweetheart every child is different just as adults have different personalities, part of this is his nature not anything you have done or not done.
I still think the teacher should have called you for a chat first too not just sending a bomb like that out of the blue.
Another thing I have thought of is that the school has received funding for a self esteem program so they have rounded up every kid who even faintly fits the bill to go to this. Do you kow if other kids are doing it too?
I have a little girl in my class who is very reluctant to work by herself (she has learning difficulties too though) I have changed my attitude to her and nevr go head on with her about anything and I sort of jolly her along or help her a bit then let her do a bit. I also have a system with her mum which at the end of each day when she picks her up outside the door I either give "thumbs up" or thumbs down for her attitude/behaviour to the day. MUm has a chart at home and every thumbs up gets a sticker, if she gets any stickers from me during the day they get added too. After the card fills she got a treat/prize. I cannot tell you the difference it has made to this little girl- she now is smiling and happy at school whereas her last years teacher said she was sullen and cranky. She just thrives on this praise, some days have even been 2 thumbs up!! (one memorable day I told her mum that if I had more thumbs they would be up to LOL) We have had the odd thumbs sideways or down at the start but not for weeks now. Do you think your DS' teacher would help with something like that??
Sorry to ramble on I am just worried about you cause its obvious you are a great mum and you are not being supported.
PM me if you want me to post you a copy of the chart or anyhting