A little flattered to hear from you suddenly? How should I answer him?

You can reply to him with a lovely expression.

I think this boy has a good impression on you, because he said he was flattered, which means he always thought you wouldn't contact him actively, but he is looking forward to your contacting him.

I want to know how you feel about him. If you don't like her, then you can reply coldly and casually. But if you have a good impression on him, you can be warm and lovely.

In fact, whether you like him or not, the content of the reply is not that important. If you don't care about him, don't care about his feelings. If you care about him, he will feel very happy no matter what you give him.