It sounds like you’re leaning more towards it being a sexual dom that you want?
If he doesn’t understand it, then you either didn’t explain it in a way he could understand or he doesn’t want to hear it and is being a bit bratty himself. Maybe he’s a sub himself.
It’s hard to explain your real feelings on something when you know that you’re not going to get what you need and especially now that you’ve said you need it in some way or another.
You recognise and are willing to say that it’s something you’re not going to compromise on by not having it. And neither you should. I would suggest being careful about how you position that to him as realistically it’s an ultimatum. You’re going to have it in you’re life and whether he stays in your life depends on his acceptance. And if he can’t accept it, then its right for him to go.
Could you sub with him and share that experience together? I think if once you talk to him again about what it is that you’re wanting from a dom (maybe your needs are not stereotypical sub needs and thats why he doesn’t understand) if its not all sexual and you basically want someone to make the decisions in your best interest. Does he understand the need behind that.
My partner and I have sub/dom esque sex, in that he gives me what I need, regardless of whether its pain or love or a mix of the two. And he gets his enjoyment from giving me what I need. We set an agenda for the time we’ve got, not an itinerary that has to be followed but if I need to be tawsed then theres a feeling that I’m needing and he’ll give me that.
I’ve tried taking on the domme role with previous partners, I’ve had whips and crops in my drawer for years and when I’ve mentioned that I’ve had them, they all assume I want to use them on them, thats annoying. I thought by taking on the domme role I could imagine the feel of the crop on my skin when it was hitting their skin. Honestly that just made it worse, that pissed me off and I had to stop because that frustration was coming out and thats not what being a domme even a fake one is about.
So don’t do what I did. Don’t compromise. Because that feeling of getting the release that you crave won’t go away.
suggested the mastubation bit to him and he’s going to give a try 
feeling more relaxed and just more me……and found calling him ‘good boy’ in a bratty tone get punishment automatically (normally ass play which still getting used to) so that’s fun for my brat side 
.
step by step as sounds like he really wants to just make me happy