No, a boy you don't know is more usually लड़का वेटा is usually son unless it's used as a term of endearment or respect about a boy\child you know or occasionally sarcasticly to patronise; actually very similar to the way "son" is also sometimes used in some English dialects.
You are saying that "my son" can be used to refer in some contexts to a person who is not in fact one's son. That's true. But I was saying that people frequently use "my boy" to refer to their son: "My boy takes after me," or "My boy is nearly ten now."
That's true, but they are two separate words, both in English and Hindi. They have distinct translations - whilst they can be informally interchangeable to some extent, it's probably not a good idea to let them be interchangeable on a language learning course.