Aloha - first of all, I am so sad that you are going through this. You know your own child, and you know that what they are describing to you, isn;t the DS you know and recognise!
Please don't think for one second that you don't know DS - rather that it is evident that they don't know him or what he is capable of, or how to get the best out of him. All of that is sad; all of it inexusable, ime
I have been to the school that Marina linked to - Browns. I went there when it looked as if my DS would not be able to be educated in mainstream school because of his speech difficulties. However, they have now been largely resolved and DS is doing very well in his mainstream school.
Browns was wonderful - truely wonderful. It is a great reputation in my area of SE london, and many SALTs recommend it. The school that my DS is now at, also recommended it as a school where many children return into mainstream school after a year or two there. When I visited them, they spoke a lot of the plan being to get children into mainstream schools if it was appropriate. I can honestly say, that if DS had ended up at Browns I would have been very happy with that outcome.
I can't believe how quickly this seems to have escalated from 'doing fine' to 'lunch time exclusion'! Your DS is one fortunate boy though - how lucky is he to have such great parents, who come hell or high water, will do everything in their power to get the best for him.
Your belief in your child and your drive to do the best for him, is your child's best asset by a long way to my mind. I have no doubt that you will use every ounce of your conviction, articulateness and intelligence to secure the education for him that he so richly deserves!