I dont see her as 'stepmum'. She's my ex's gf and whilst she may be 'nice' on the odd occasion I see her, I dont know her very well and I am not comfortable with her having sole charge
OP, I don't know how to say this very well, so please know it's meant with care and not any bad intent: I am so so sorry, but you no longer get to make that call about whether she is his stepmum. It isn't about what you perceive her as. It's what your son perceives her as that matters.
I may have misread the situation, but during your exes contact time, he can leave your son with anyone he feels is responsible. Obviously, him encroaching on your contact time is unacceptable but a different matter.
Your son sees OW's daughter as his sister, and her cousins as his cousins. So it's fair to say that to the outside world, the OW IS his stepmum. I know that must hurt, but you will always be his only mum.
I am only saying this because if you broach the subject to your ex in an "I see OW as your GF not DS's stepmum" it could play right into his power games. Because he can argue that she is and, after two or three years together, your son is bound to see her that way.
I am so sorry 

Also, I would be aware of people projecting their own experiences onto yours. Lots of people display some narc tendencies, but diagnosis of NPD is still pretty rare. I am always wary of people diagnosing over the internet, albeit with the best of intentions.
I'm an ex MH nurse, and nursed a few narcs, but the really severe ones are few and far between. Narc and arrogant behaviour, together with behaving in a way which they think will make them seem wonderful, is a facet of another well known disorder that is rife in married men who have affairs: the disorder is known as "being a cunt". I have had FAR more psychiatrists diagnose that over the years then narcissism (not to the clients face, obviously!).
For now, I would take his behaviour at face value, but keep in the back of your mind what others have said about narc behaviour. Don't let your exes motivations cloud your thoughts or he is in your head again. Keep them in mind but focus on you and your son.
PS Your ex sounds like an absolute bell-end. That's a disorder I saw a lot too 
