I think it is an interesting study to explore the correlation between Kageyama's hair and the comfort level of his emotional state in the context of volleyball
Let's start with his bowl cut hair, which appears at the start. He's learning about the world, and he experiences the joy of learning more volleyball and taking on challenges. He has a support system in his grandfather Kazuyo. A carefree child.
His hair is shown to be slightly longer than his usual short, bowl cut the first time in this panel. His middle fringe starts appearing prominently from the point on when he has lost his grandfather. The loss of his support system and the onset of a lonely journey.