Send a message to a boy to apologize, but he won't reply. Why?

First, when you say "sorry", it means that you may have hurt him before, which is a deep injury. Maybe an apology or two is not enough to smooth over your injury to him, or your apology is not sincere enough.

Second, maybe this boy is narrow-minded and doesn't respond when he is hurt, so he just ignores it.

Bottom line: apologize sincerely, be yourself, treat each other sincerely, and have a heart in mind.