Soph year my son was cut from basketball after being the freshman point guard. There were people who thought he might make the jump to varsity. He was cut for missing almost every “optional” workout and not playing for the summer team. He was playing two other sports and fall ball. Plus in high school baseball became a year round effort with either skills or physical development.
Not playing basketball allowed him to focus on baseball all winter. He made a huge jump in skills the first two winters not playing basketball. He also bulked up rather than losing weight running the court.
Your son has to do a little soul searching on how much he will miss basketball. But if he’s committed to putting in the work after school every day for baseball and wants to do it I would let him. Offseason workouts need to be something organized he does with other teammates so he doesn’t goof off.