‘He is polite’ means that he is generally polite, it refers to his general character. ‘He is polite’ means that he is a polite man. When we say that a man is polite, we know that he is and has been polite. It is not a temporary state. What we mean is that he is polite by nature.

‘He is being polite’ means that he is behaving with politeness. It refers to a particular act of politeness at a particular moment. It doesn’t say that he is innately polite.
When we say that a man is being polite, we notice that he is polite at a particular moment. This refers to a temporary state, a condition at a special moment.
Other examples:
He is being kind.
He is being clever.
He is being friendly.
He is being rude.
You are being ridiculous.
She is being unreasonable.
She is being frivolous.
He is being naughty”