"sport" is pretty much a friendly way of referring to someone. the term originates from a book called "The Great Gatsby." these days, is can also carry a sarcastic/condescending connotation depending on the tone.

Yes, in that context, it means buddy or friend. To me, it is somewhat formal and more British. Americans do not normally use the word "sport" as a salutation. In another context, the word might be used to address someone unfamiliar, as "sport, can you tell me the way to the post office?"

sport = buddy

It's not solely American - it's used in Australia as well (any input from the UK?). A "sport" is a person, but when we say "(s)he's a good sport", it means that person is fair and agreeable... maybe because if they played sport, they would play it well and fair. :)

You are right. In this context, the word "sport" means buddy. I picture the person saying this as an middle-aged American man.