I'm no expert, but ADHD is supposed to present itself in all environments, so if he's fine at home, at restaurants, and temporarily in a new school environment, that's a red flag.
You mentioned your therapist gave the ADHD diagnosis in part because of answers you gave about how your DS behaves at home... I think it's worth exploring this with the therapist, because apparently she heard something in your answers other than what you expected. Maybe she's catching on to clues that you're missing, or maybe she's misunderstanding your responses.
I also note you say that you and your DH are both introverts, your DS is an only child, and he has no regular exposure to peers outside of school. Given all this context, it would make sense that he would be socially underdeveloped, because where is he supposed to learn it? All of the social misbehaviors you're reporting are normal in younger children, and they learn to work through these issues primarily through play, with both adults and peers.
As for impulsivity... he's a second-grader.
I'm not saying he's not ADHD, because again, I'm no expert. But I am saying there are alternative explanations for all of these things that merit further investigation before accepting that label.
