If a Protestant believes he cannot call a priest "Father," what respectful alternative can he use?
The short answer
He can use the title he gives his own ministers; most priests would not mind "Reverend" or "Pastor." If asked to say "Father," he can politely decline, and the priest will understand.
A Protestant who reads Matthew 23:9 literally could use for Catholic priests the title he uses for his own ministers. Most priests would not mind being called "Reverend" or "Pastor."
If the priest says, "Call me Father," the Protestant can smile politely and say, "I'm sorry, I can't." The priest will understand.