A son is a male offspring; a boy, man, or male animal in relation to his parents. The female equivalent is a daughter.
In the vernacular English of the United States, the term is also one of endearment between young Black males, similar to "homey" or "nigga", although it can also connote a sense of superior standing on the speaker's part, as the word carries a link to references of age and, therefore, the speaker's seniority.
Most fathers and mothers in preindustrial societies attach high value to conceiving a son; often a son is considered more valuable than a daughter. The reason for this is that in those societies, males have higher social status, and are physically stronger.
He is my son. I love him.