Revisiting Mound v. Flat Ground Training

My son is 17 years old and has been a starting SS and pitcher for his HS team. Every summer he plays American legion and has attended various college camps. The issue I need addressed is that his pitching control in the summer months during non-HS activities is good to excellent. Once he starts the school season (his HS plays fall and spring BB) his control deteriorates significantly. The only thing I can think of that may be contributing to this is that during the summer he pitches almost exclusively from a mound for practice but once he begins the season in HS, pitching practice is primarily constituted of flat ground drills consisting of once a day 20 pitch sessions. The pitchers are only allowed to throw 3/4 speed with fast balls and changeup. No other off speed pitches are allowed. The pitchers are allowed to pitch one live inning in intrasquad games from a mound each week if the pitcher isn’t due to start within the next 3 days. I suspect the HS coaching staffs belief that this practice regimen reduces injuries is the reason it is used.

I am of the opinion that the difference in mechanics between mound and flat ground practice is the biggest cause of his control issues in the fall and spring v. summerball. Some of the other pitchers have experienced similar control issues albeit to a lesser extent than my son.

I am trying to get other points of view on this subject and recommendations on drills he might use to correct the issue.

A lot of different variables to consider.
On a simple viewpoint with physics, a mound is an incline plane. Incline planes are simple machines used as a force multiplier. For training, I like to stay on flat ground and work mechanics to reduce unnecessary force on the arm.
Another variable is the mound itself. What are the conditions of the mounds between the two leagues. Is he throwing on an artificial mound in the summer, and a dirt mound with the HS? The mound conditions could be forcing him to change something in his mechanics. If you have video, check to see if something is different in his delivery between the two leagues.
Last, nothing is stopping you from getting in some side work during the HS season. An extra bullpen per week might be helpful, if you know his pitching rotation. Get a good warm up, then go through all of his pitches 5 each in progression. I like to start the progression in the stretch and end with 5 four seams from the wind up.

Thanks. I will share the information with him.