For better or worse, he needs to listen to the coaches. They in fact have his best interest in mind and success as their goal.
That said, however, there is no guaranty of success even if he makes those changes.
If he makes the changes and succeeds, that's the end of the issue.
If he makes the changes and it doesn't work, he can either return to his old style in summer ball or try a new style in summer ball. If he succeeds in summer ball with his old style the coach will probably tinker no more.