son
In German, the word "Sohn" is used to refer to a male offspring, just like the English word "son". It is a noun and is capitalized in German. It is often used in family contexts and can be used in phrases such as "mein Sohn" (my son) or "der Sohn meiner Freundin" (my friend's son).
My brother's son is my nephew.
In this sentence we see a family relation being explained using the word 'Sohn', which means 'son'.
She is the mother of my son.
The speaker is indicating the relationship of a woman to his son.
My son is eight years old.
This sentence is a simple statement of fact about the speaker's son's age.