Hi Colinsmum,
You're absolutely doing the right thing for your son from what I have read in your posts

I feel like this so often - that my friends who do know about dd think I'm making a fuss about nothing. Dd barley speaks in the presence of adults she doesn't see VERY regularly, so a number of them would be lucky to have heard her say much more than a sentence.
I think you hit the nail on the head when talking about trust. It would be wonderful for even just a few people to trust that you're really making your choices based on the best information you can find and that while they're often not perfect choices, particularly when you have so few options, you can only do what you can do. It would be so much easier to be trusted instead of having teachers, friends, family giving you that tight little smile where you can see they're trying to humor you but you know they think you're nuts.
Sometimes I am particularly struck by the lack of fairness we experience raising these kids, and yet at the back of my head I have a voice that tells me I have no right to complain. My daughter has amazing potential, I should pull my head in right?But having to constantly censor while other people speak about their pride in or concern for their kids is boring and exhausting and lonely.
I feel very lucky to have finally won my husband around who was doubtful about dd's abilities until she was tested (he's always much better once he has an 'expert' opinion), and to have one wonderful friend who gets it. Otherwise I feel much the same with many of my other friends as you do with the friend you mentioned. Please don't take this as advice, because I hate never feel qualified to give it, but for me I have decided to keep a number of friendships and just push on through. I don't go in to much detail, but I don't avoid the topic or deflect questions. I figure the more we talk about it as normal the more it's out there in the world as normal.
I'm sorry you've had that experience though. I'm not sure the fact that it seems to be a common one is much of a comfort! Thank goodness for this board.