I really have nothing to give in form of advice or experience (although I could see my son, 4, being in a similar situation one day). But I just wanted to reach out and say that my heart aches for your little guy. It is like he wants to do some things differently but cannot figure out how. And you're right that might be a maturity issue. But the feeling like you aren't quite fitting in, I don't know, I think that might just come with his being different in terms of his thinking, maybe being very sensitive. All I can say that from your posts I feel like you are the best champion he can have. I sometimes say to my husband that it feels like our son chose to be our child because he knew we could handle him. We could keep him safe and make him feel good about his quirky, wonderful self. That we could get the greatest happiness from who he is. That's why we were given the task to be his parents. And it seems to me that you are doing just that, too. It is exhausting, of course. And the first time I saw my son hurt because others thought he was odd because he is very outwardly happy when he is happy (something we love about him!) was incredibly heartbreaking.
So no advice from me regarding the school situation (other than I agree that a teacher should make sure kids don't get excluded in the first place) but just a virtual hug to you!
Take care,
Stefanie