A polite/honorific way to refer to someone else's son.
田中さんの息子さんは医者になったと聞きました。
I heard that Mr. Tanaka's son became a doctor.
お宅の息子さんは何歳ですか？
How old is your son?
息子さんは日本語がとても上手ですね。
Your son is very good at Japanese, isn't he?