Students don't reply to text messages on their birthdays.

Maybe the purpose of his reply to you is only to respond politely to your blessing, not to your previous questions. In addition, people may behave differently in different emotional states. Maybe his emotional state at that time was not suitable for replying to your previous question, but when he saw your blessing message, his emotional state might be more relaxed or positive, so he will reply to you.

If you are confused or dissatisfied with his previous behavior, I suggest you find an appropriate time to talk frankly with him, let him know your feelings and thoughts, and see if you can solve the problem. But you should also pay attention to your mood and tone, and try to avoid using too negative or aggressive words.