Sound guy here.
Soundchecks are for setting a mix up. You can do that at any volume.
Mostly for drums and vocals.
When I Soundcheck a band I set a drum mix up then sub-group it. If when the band starts playing I need more volume I just bring the sub-group up.
Same with vocals.
But that being said it all can be done on the fly and should end up sounding just as good.
But I do see what you are saying and It seems that this sound guy has a case of soundguyitis also known as grumpydudewhohateshislife disease.
Tell your son he's doing a good job and to never let a sound guy get to him. We're a harsh bunch.