Paladin -- I don't think anyone who has posted here believes that your son should not have had a "discover scuba experience" or that, quite frankly, given all you've written, that this was a particularly dangerous experience. (Note, yes, he could have embolized had he taken a deep breath and shot to the surface but that is true no matter what.) The comments relate not to you and your son but, to the contrary, to the instructor.
Like it or not, there are "standards" that all "professionals" are supposed to uphold. This is true regardless of the profession. Given all that you have said, I do not believe this particular instructor upheld the standards of his profession.
What he did may well not have been any more dangerous that what I do showing friends how much fun it is to "blow bubbles" in my pool. But THAT is not done in a "professional" setting and it does not put my LDS at risk (however slight).
Anyway, go off and think about what idiots and uptight people we are if you desire but, just for a moment, try to think about it from the aspect of "What if....?" Would you not then be asking yourself, "Did I really do the right thing and would it have been better if my friend, the instructor, had actually done what he was supposed to do?"
And, just to be a bit harsh, as a father who has buried a child, I can tell you that you NEVER forget that moment!