Well done, Tan it sounds like you have handled the MIL situation perfectly. I can see that it was a knee jerk reaction on her part, not wanting to believe anything is wrong with her grandson! Completely understandable. And as far as how you're managing his behaviour, well you are just doing the best you can with everything you know about it. Hopefully the doctors can help you get to the bottom of it, so that you can find strategies that are really tailored to what he is going through - once you know what that is.
As for why he doesn't trust you - I really don't think that is it. I think he does trust you - look at how he relates to others and then compare it to the way he relates to you, you are probably the ONLY person in the world he does trust. It's just that his base level of trust is very different from the norm.
I really hope you can get some answers so that you can stop doubting yourself and see that you really are doing a great job in a very difficult situation