I would like to express that an old man is abused by his son. “Descendant(s)” would be used when there isn’t a more precise way to describe someone, or it is irrelevant. His son abuses him. He created a trust to care for his descendants. (All of them. His children, his grandchildren etc.) The idea that ‘descendants’ have to be distant isn’t correct. Example: If you die without a will, your estate will pass to your surviving spouse, your lineal descendants and your collateral descendants. ‘Descendant’ can be used for remote relations, though. Genetic analysis shows that every European is a descendant of every 9th century European who left descendants.

Hi Hua Yang, As you can see the LDOCE5 says, "someone who is related to a person who lived a long time ago, or to a family, group of people etc that existed in the past → ancestor". so we usually use it to talk about someone or people who existed a long time ago.

Your suspicion is correct! We would not use 'descendant' in that way. 'Descendant' is the "opposite" of 'ancestor' and is used when referring to family relationships over a period of several generations. ............... Edit: NB The exception is in certain specific legal contexts, when referring to matters of inheritance (see DavidK's example). The language of the law has its own rules and conventions, and the meanings of terms as used in legal contracts and suchlike are different from everyday usage.

"Descendants" typically are not alive at the same time as their ancestors, although there's no strict rule on it. In a normal conversation, we would use the very sentence you want to express: "The old man is being abused by his son." Although saying it in that way is also somewhat passive. It's more likely we'd say something like, "That kid is abusing his father!" But if you want to increase your vocabulary, I would suggest the word "progeny." That is, "He is being maltreated by his progeny." Although, again, most people wouldn't talk this way. You might read it in a book, especially if the author is writing with flowery language. But no one really speaks that way.