So, you’ve kind of supplied your own answer here.
‘James honey, I need to tell you something. When you pulled out last night I was just about to cum. I was so close, but then I didn’t and… god maybe I shouldn’t tell you this but, it’s made me SO horny today. Seriously, all day, it’s like I’m on a high. Fuck, the things I want to do to you. I looked it up, it’s called edging or orgasms denial Apparently it’s really popular. Do you think, maybe, we should see what happens if you keep not letting me cum?’
Finish it off by begging to go down on him. And just go that bit further than you normally would. Swallow when you would have spat, get your tits out, edge yourself as you do it, suck his balls. Whatever. Just plant in his head, ‘Holy fuck, if Nat’s like this after a day what’s she going to be like after a week?’
That’ll work. Although, guys are idiots. Never underestimate this. It may take a few repeats of this until he links not letting you cum to how horny and wild you get. But give him time, he’ll get there.