Not that the rational helps in all moments, but it may be worth pointing out to him that when minimal efforts yields a strong reaction, people who are borderline bullies feel even more inspired. Or if your buttons are easy to push people will line up to push them.
As to the other parents' remark, I would try to figure out if the root is their own observation, something their kid told them, or something the teacher said. Mostly because if the teacher is saying something to the class that can lead to a fairly hostile environment.
