The main thing after shutdown is a SLOW ramp up. Start w/ stretching, band work, Playing catch for 5 min day 1 at 60 feet. Then maybe 7 min on day 2........It takes a while to get back on a mound.
I like Alan Jaeger. Alan always talks about working the calendar 'backwards'. In other words, if you want to throw in a game on Sept 15 , you work backwards from Game day w/ a pre-hab throwing program that ramps the player up where they project to throw a pen a week before the game then again maybe 3 days before the game.
Here is an article he wrote about ramp up. It is centered around long toss but you can get a good idea of timelines regardless of whether or not your son does long toss.
https://www.jaegersports.com/j...on-throwing-program/