Can your son contact them instead of you? I did sometimes call admissions but only because my daughter was at school and working etc. and then I still felt it was kind of inappropriate. Maybe your son doesn’t have time…
I don’t think you need to worry about any of this. He should list the other schools. The resume may not matter, for the reasons you state (but your son can double check after he sends in the application, because chances are it would have to be sent separate from the application even if allowed). He can put an asterisk on masterclasses: it really doesn’t matter because they will see that it was a masterclass, right? So yeah, he can go ahead and mark that as a performance I would think.