I think the term comes from back in the day when men used to herd cattle and most of the cattle were cows and steers. A steer is a male bovine that is casterated before maturity. So I think that it was easier to herd steer rather than bulls so that is why they are call cowboys. Now bull riders are not called bullboys due to it being a rough sport so it would sound wimpy to call someone a bull boy hence or emphasis boy, when it takes a man with a heart and a lot of guts to crawl on one.