This is called a mixed conditional, and you need to ask exactly why the speaker chose to start with a second conditional then finish with a first conditional. If you understand the rules clearly for English conditionals, you'll be able to identify why the speaker chose to mix them.

This is not correct because it doesn't make sense with both a past tense and future tense verb. If you want the future tense, it should be: "If I meet him, I'll tell him." or "If I see him, I'll tell him." where both verbs are future tense. If you want the past tense, it should be: "If I had met him, I would have told him." or "If I had seen him, I would have told him."

If I meet him I'll tell him.