Um-- I'd tell him that it is rude/presumptuous, and why. Who HASN'T thought something along those lines? The difference is that most people have the social grace not to SAY it.
If he insists that he should not need to regulate his speech so that he doesn't SAY all of the things that he THINKS, he is going to be treated accordingly by people who have witnessed the behavior. That's the consequence of not keeping your mouth shut, basically, when you don't have anything nice-- or at least USEFUL-- to say. It's also a consequence of not respecting (or at least being wary if you don't understand) the social constraints that govern particular relationships. (Doctor-patient, Parent-child, Teacher-student, Boss-subordinate.) There is a reason why Confucianism had pretty rigid expectations there.
What does he feel that his commentary accomplished? He is basically thinking that he knows well enough to
critique the job performance of professional adults, correct? In other words, at 7, all he knows is that he is smarter than most people, and he has (wrongly, as it happens) concluded that it means he knows more about pretty much everything than most people.
Those people work pretty hard to be hearing it from a 7yo who-- no matter how intelligent-- lacks germane life experience. His cognitive abilities do NOT render him judge and jury of all humanity. He doesn't know the full picture because of that lack of life experience as an adult; smart
er not to make snap judgments on the basis of data he doesn't possess.
I might even point out that a truly WISE person knows enough to know that there are things that they don't know-- and that some of those things probably matter. That's why it's often better to bite one's tongue when a flippant or arrogant remark comes to mind. (Not that I know about that...
)
The sooner he learns that lesson, the better off he'll be. I think that it sounds (judging from his responses to your very gentle redirection thus far) as though it is time to be a little more blunt about that point.
My son understood, but also asked if I didn't think it was a little bit funny, too.
My answer to this kind of thing with my own child-- at ages younger than yours, btw:
Why? Do you think that {person targeted by comments} found it amusing? How do you think you would have felt if you had been him/her? 
