A polite/formal term for (someone else's) son; often used in written or formal speech (commonly as ご子息).
社長のご子息が今度入社することになった。
The president's son will be joining the company this time.
ご子息のご健康をお祈り申し上げます。
I pray for the good health of your son.
彼は訪問先で相手の子息について丁寧に尋ねた。
He politely inquired about the other person's son during the visit.