There is no way anyone can tell you how effective your son will be facing varsity baseball players. First of all we have no idea how good the competition is he will be facing. Secondly we have no idea how good of a pitcher your son is. And thirdly he could be very effective against one team and get hammered against another. The talent level from batter to batter and team to team in hs baseball varies greatly.
From my experience there are many varsity level pitchers who pitch in the upper 70's to 80 that have a lot of success. If - They know how to pitch - If they can locate their stuff very well - And they have a breaking pitch that is good they can locate for a strike in any count. I guess that goes along with knowing how to pitch. And of course they have a catcher and or coach that understands how to attack hitters.
In hs most teams will only have a couple of truly advanced hitters and some none. You will not be able to throw a 79 mph fastball by a hitter sitting on a fastball but you can get him to roll over it or miss it if you have set him up properly. A pitcher that can command extremely well and command an off speed pitch as well does not have to throw mid 80's to be effective at the hs varsity level. The fact is if the truth be known there are more hs varsity pitchers pitching in the upper 70's to 80 than there are above that velo. Now others may have other experiences with this that tell them otherwise. But from my experience this is the case.