Yah no I would be dropping him over this. There's no rebuilding trust because he lied to you and hid it from you.

If he had stopped at the airport and getting her number for educational reasons, fine whatever. I would have been mad and jealous but moved on.

This isn't just borderline cheating, I would consider this cheating considering the flirting and offering to take her places that he knows are romantic or you found special.