Honorific term used to refer to someone else's son, typically in formal or polite contexts. Not used for one's own son.
Which university did your son go on to?
令嬢 is the honorific term for someone's daughter, while 令息 is the matching honorific term for someone's son.
令息 is an even more elevated, formal term for someone's son used in written correspondence, while 子息 is already polite but somewhat less ceremonious.
(someone's) son
Compound of 令 (rei, 'good; excellent; respectful prefix') and 息 (soku, 'son; breath; rest'). The kanji 令 is used as a respectful prefix in formal terms, while 息 here carries the meaning of 'child' or 'offspring'.