First time Stepmum, no kids of my own.

[FatParrot]

Guidance pls, interwobble.
I’ve become involved with a man whose 14yr old son lives with him full time. The situation with the child’s mum is (v) complicated, although that’s not why I’m posting.
I’ve had a stepmum myself since I was 11, so I know how things work, but never done this myself.
I’d like some guidance, pls, on how I should boundary myself with his son. With my nieces and my friends’ children I have a longstanding auntie role, including telling them off for doing silly/dangerous things, not being cheeky, manners, refereeing arguments, etc. I understand that this situation is very different, however. I don’t want to ‘mother’ him; he has a mother that I’m not trying to replace. What should/shouldn’t be my responsibility.
We don’t live together, but I stay there multiple nights a week. TY in advance.

I’d pick your battles OP. If he’s a rude and inconsiderate little shit then at least he’s not your little shit. Just limit the time you spend with him and hopefully he’ll leave when he’s 18.

It can (sometimes) be horrible living with a 14-18yo son even when he is your own flesh and blood. It's a really difficult time. Even a well-balanced teen living with his own parents will regularly be stroppy, resentful, resistant to any kind of rules or regulations, selfish, careless and arrogant. And then they grow out of it. Even during the worst of times you do get occasional glimpses of the sweet and vulnerable little boy he used to be and the mature and independent young man he is becoming and the confusing changes in brain and body that are at work in him to achieve this change cause chaos that everyone around suffers from. Whatever you do, do not move in with your boyfriend. When you are staying over, be prepared and able to leave at a moments notice if necessary. Do not try to be his mum. Expect and require civilised behaviour but if you don't get it you leave and go back to your own place and let his dad deal with him.

Meant kindly but you are not his stepmother. You are his father’s “girlfriend” for want of a better word.
It’s not your business in any way at all to intervene in anything.
Leave his father to parent his child and you’re basically dating the father. Thats it.

My advice is don't do it, don't get involved. Get out now. Please see Reddit step parenting forum for reasons.