I agree, to change armslots against a batter or within an inning is very difficult. He'd have to have incredible command and velocity to switch and remain effective. Having a sidearmer pitcher that has to prove his spots to the umpire, I wonder how many of his close throws deep in the count would be called balls because the ump hasn't seen him throw that way or hit that spot before.
Leftyside tried this in high school. He'd throw a couple overhand and then switch to his more natural armslot (sidearm). The problem was that he didn't throw hard enough and so batters learned to swing on his first two pitches before the nasty stuff came. He eventually learned to stick with the nasty stuff and ended with a 1.33 ERA his Sr year and is now pitching strictly sidearm in college.
Why does he want him to do this? Has he been effective as a submariner? Nearly every team(D1) our son has faced this season has had either a submariner or sidearmer in relief. Leftyside was the go-to pitcher out of the bullpen this season with a batting against average of .178 He still does not throw hard but he is nasty!
Does your son want to play in college? This could make him hard to hit in high school but not much of an asset in college because he hasn't been able to fine tune one area. IMO, Better to be excellent with one armslot than very good with two.