Love summer time,
First of all, good to hear your son's velocity is exactly where it should be. Don't be concerned if you hear of some stud throwing 85-86 at the same age. There were a lot of such kids when Bum, Jr. was that age, and he's passed most of them. He's 90-91 now, cruising 89 or so. Not bad for a little guy.
I agree with most of what BOF says, especially about leg and core work. I believe Fall is the ideal time to back off a little, and focus on the mechanical issues, pitch development, etc. Of paramount importance is getting a qualified pitching instructor and working on delivery (not bullpens). (And no, not the Summer coach or a guy who just started up a new sports academy.. Ideally I mean a college pitching coach.)
The cardio and weight lifting should be maintenance-only.. start the more serious stuff in the Winter.
In the fall, your son's energy level after a full season is undoubtedly quite low. Back off, feed him well, and let him be a kid for awhile!
My only disagreement with BOF is the shutdown aspect. Bum, Jr. NEVER shut down from long-toss, throwing year-round, and he had great success in that approach. This past year he had to shut down twice, for the first time in about eight years.. once on his coaches advise and once when he hurt his ankle in college ball.. I know he didn't like not throwing!
Good luck.