Will it affect boys to send him a message to cheer the day before the basketball game?

Sending a message to cheer a boy the day before his basketball game won't affect him.

If he is interested in you, he should be very happy to receive the text message. If he is good at it, he should invite you to watch his basketball game. The influence on him is also good.

If he is not interested in you, he won't care about you at all, let alone the information you sent, and may return a thank you out of courtesy.