Polite way to refer to someone else's son. Not used for one's own son; for that, use 息子 (むすこ) without さん.
How old is your son?
I heard that the neighbor's son passed the university entrance exam.
息子さん is the polite term for someone else's son, the male counterpart to お嬢さん for someone else's daughter.
息子さん is the polite way to refer to someone else's son, while 息子 is used plainly, especially for one's own son.
Formed from 息子 (むすこ, 'son') + the honorific suffix さん.