I’m dating a cis dude and when I brought up poly he accepted it but now he’s asking if I couldn’t just “stop”? I can’t and I don’t know how to go about this.

If your partner wants you to “stop” being or doing something that you can not or will not stop, you two are at an impasse. 

Explain to him that polyamory is important to you, and dating you means accepting those terms. If he decides not to date you because he realized he doesn’t want to date on those terms, that’s his informed choice.

Just because someone said they were okay with something in the beginning doesn’t mean they aren’t allowed to change their minds. That’s what dating is all about: you spend time with someone and get to know them so you can figure out whether you want to commit to them long-term. It sounds like he gave it a try and doesn’t like it. That’s his right. But him realizing that means he has the choice to end the relationship now that he realizes it isn’t working for him - not that he gets to demand major changes from you.