The two bigs things you should take away from this are that you are 100% correct that as soon as parents get involved it will end in tears no matter who is right/wrong.
The second thing is that your son is entitled to a ball mark inspection as it is on clay. If he is confident of which mark he called out and that mark is consistent with the type of shot played then he should get the point as it is his call. If the supervisor thinks they have seen the ball go in then they should be able to point out their mark.
If they refuse to do a ball mark inspection then your son can call the tournament referee. Then a polite, respectful and well argued case from your son that shows a knowledge of the rules will go a long way to having the point awarded to him.
I know it is a fine line between not getting kids too worked up over these things and not giving in to bullying behaviour. For me in the end it comes down to what my kids feel comfortable with. And I know if it was a tiebreak in a final they'd kick themselves for weeks if they didn't stand up for themselves.