Yes, that is a good analysis. "He is polite" describes his character, while "He is being polite" describes the current situation.

Your guess is exactly correct. "He is polite" is permanent, (He is a polite person) and "he is being polite" means that the person is doing that currently. Examples: Fred is polite, he always says please and thank you. Robert is being polite, he opened the door for me today.