From what I gather, your son is a 2020 who missed sophomore season due to injury and then played last spring, summer and now, fall. I assume the Mar 15 date is the start of HS games? I'm sure some rest is in order but I think you have to work it back from when he will actually need to have his arm ramped back up. There is too much we don't know. When will his HS program start throwing? Is he a P only or will he participate in all the practices and drills for position players? When will P's begin bullpen work? Does he have or will he have an arrangement with HS coach to ramp up later than others due to needed rest coming off of playing so much post-injury? He obviously can't go from rest to jumping on the mound in a game on Mar 15th. Nor can he jump into team practices or pens where there will be significant throwing expected. At least, he needs to find out when team activities begin and ramp up his arm in time to participate in those with a strong healthy arm that is ready to take on a throwing load. Of course, sequential steps need to be taken between starting back into throwing and throwing the first fastball only off of a mound... and then more steps before adding secondary pitches.
As far as velo programs, I don't know of any that don't include some element of high intensity throwing so that can't be done during throwing shut down. I think a proper weighted ball program and certainly long toss are a good part of any ramping up and maintaining of arm strength and health.
