The best approach is to just be honest and tell him. Send him your own version of this message.
‘Honey, I had a special request for the next time we’re together. I love how you make me cum, and you’re really good at it. But I have this really strong fantasy that I’ve not told you before – I don’t want you to let me cum. Not at all. I want you to get me as close as you can, and then just keep stopping. Keep teasing me. And no, not even at the end of the night, just leave me like that, dripping, horny, desperate for you. I want to see how it makes me feel, I think it’ll make me even more of a little slut for you. It’ll make me even more desperate to please you, service you, let you do whatever you want to me.
So can we try honey? Leave me like that, keep me like that, maybe even the whole time till we’re next together? I’d love that so much, I hope you would too.’