PG,
No doubt for a showcase, 77mph won't get any looks, but part of the OP was related to son not getting mound time on HS team. The points made related to 77 not being a horrible velo, was directed by those who commented about your average HS kiddo....I've seen crafty HS pitchers succeeding at the HS level throwing upper 70's, but certainly not at showcases. I wasn't clear whether the OP was contemplating missing a HS game to go to a showcase as a pitcher or position player....although arm strength would likely need to be higher even for a fielder .
I would also advise not toeing the rubber in a paid showcase with a top velo of 77.