Hm, to demonstrate the liberties with honorific translation that I've taken, here's a few examples.
Take, for instance, Master. As I mentioned earlier, it can mean shifu (師傅/師父), a catch-all title for a respected teacher and usually head of a school.
It can also mean laoye (老爺), which I would translate as Old Master, and is generally used to refer to the patriarch of a clan or organization, and denotes a certain level of higher social status, be it through wealth or aristocracy. Zhou Dingqiu of the Zhou Manor would usually be referred to this by his servants, while his peers and disciples might still call him 'Master' in English but actually mean 'shifu' due to his prowess in martial arts.
Following up on it, we then have shaoye (少爺), or Young Master, which is used for young scions of a clan. Here it gets a bit more complex, where I have used Young Master as a translation for at least three different titles.
Again, status applies here, along with a sense of respectability. For example, Zhang Manxing can be easily called a 'shaoye' everywhere, thanks to his clan's influence. Bai Jiutian would pass as a 'shaoye' too.
Xu Jing can't. At the most, he can be called a respectable gentleman... politely, that is. He doesn't have the clout to be called a 'shaoye'. His peers - again, if they're being polite - would use the term gongzi (公子), or more literally, son, though in this wuxia genre it is what passes for a formal Mister for young, dashing, well-spoken males like he is. Most females will use this as a form of address, at least until they become more intimate with you and/or realize you are a lout that does not deserve to be addressed respectfully. The term for Mister, xiansheng (先生), on the other hand, is more commonly used for one of a more mature age.
I thought of using 'Sir' for this, but that carries western connotations of rank and nobility which does not truly apply here, in my opinion, as well as not demonstrating the age context.
Another term, which is rather more exclusive to the genre and which I've also translated as Young Master, will be shaoxia (少俠). It loosely translates to Young Knight, or Young Hero. You can see how awkward it'd sound if people call you Young Hero Xu, while Young Master is something you'd be more used to, but it is a term used specifically to refer to up-and-coming, young, powerful, heroic martial artists. If they are feeling particularly diplomatic, or want to flatter you, or are just really, really big on the whole gallant orthodox thing, they'll use this.
'Sir' could be suitable here, but again the age-relevant context is lost, in addition to the flattering implications of respectable martial ability and honourable conduct.
Of course, if you actually go back and comb through the updates, you'll find that I haven't maintained full consistency for these translations and might have gone with whatever sounds better at the time.