Consider programs like STEP at Purdue, just one week - not to get him into Harvard but to see if something STE, specifically engineering will even work for him - if he’s able to be accepted. I was wrong - need to be a rising senior. Put it aside for next year.
Or Operation Carapult which he could do now at RHIT.
Prep PSAT - sure but for the entire summer - heck no. 30 mins a day - great.
I think summer activities are fine.
School is not necessary. How about a job ?
Let a kid be a kid.
https://www.rose-hulman.edu/admissions-and-aid/early-planning/operation-catapult/index.html