In my opinion, no man on Earth can be called “holy”. As children of God, we can strive to be virtuous in our thoughts, words, and deeds; but we can never be called “holy” because we all fall short of the standards of God.
We read in Matthew 23:9:
And call none your father upon earth; for one is your father, who is in heaven.
Here “your father” means your spiritual father. Jesus clearly told us that our spiritual father is in Heaven. So there is only one whom we can call “Holy Father”, and He is God, our Creator.
The Pope is just a man. He may be the head of the Catholic Church, but he should never be addressed to as “Holy Father”, “Most Holy Father”, “His Holiness”, or “Your Holiness”.