The implicit question is: is your grandson's punishment disproportionate to his mistake?
Well, consider that if you're not seeing eye-to-eye with the instructor, one possibility is that your perception of the punishment is higher than the instructor's, another is that your perception of you grandson's mistake is lower than the instructor's.
For starters, let'...
I don't think you can categorically judge taking a belt away in response to a behavioral situation as appropriate or not, anymore than you can judge throwing a roundhouse kick in response to a combat situation.
Many people have pointed out that doling out punishment in order to curb undesirable behavior is ineffective. But ...