The boy said: In the future, you can tell me anything. Is it serious or polite? Is it flirting?

Whether this boy is serious or polite needs you to judge according to his usual content and tone. We can't judge who he is just by this sentence.

If you think he is trustworthy, you can tell him about yourself. If you think he is just perfunctory or teasing you, then some words are better left unsaid.

Anyway, safety is very important. For strangers or people who are not very close, it is better to keep a secret.