This has alot less to do with freedom of speech than it does with smart business.
He doesn't have a constitutionally protected right to cost your business money by upsetting other customers. If it were a government operated airline, he'd have a valid complaint. Since it's a privately (or even publically traded, because public does not equal state) owned company, they can treat their customers as they see fit. If it's bad treatment, you'll lose customers. That simple.
If you come to my office, and upset my customers, you'll get one chance to apolgize and conform, or you'll get shown the door. If you come back, you'll get shown the inside of a police cruiser.
In the great state of Texas, we reserve the right to refuse service to anyone.