I identify as polyamorous and told my partner before we met. He is monogamous and we now live in a monogamous relationship but he always says I’m going to cheat on him because I identify as polyamorous. He says I’ll either cheat on him or I’m lying about being polyamorous. I’ve never cheated or come close. How can I help him to trust me?
It’s not generally possible to change other people’s worldviews or beliefs. There exists no magic word or spell that can change his mind. It sounds like you’ve already explained this. More importantly, you’ve demonstrated it with your actions - by never cheating on him or even coming close.
Usually, when people cling to beliefs with no evidence to support them, there’s a reason. Holding onto this belief that you’re going to cheat on him serves him in some way. Framing you constantly as the “inevitable cheater” means he gets to keep you on the defensive, always having to beg him to trust you, having to plead your case, to prove your trustworthiness. And it keeps him as the ever-aggrieved party, which is a special kind of high ground that can be difficult to cede.
You’d be within your rights to refuse to engage with this argument any further. When he accuses you of being a future-cheater, you can say “I have not ever cheated on you, and I have no intentions to do so. I’ve given you no reason to say these things about me, and I don’t appreciate you constantly questioning my faithfulness. Please stop saying that.” Then, disengage. Don’t argue, wheedle, explain. Just refuse to be talked to like that.
If he continues to bring it up, or hold it against you; if he uses this to make demands or push you around, ask yourself whether you want to stay in a relationship where you constantly feel like your statements about yourself are not taken seriously and your promises aren’t believed. Do you want to date someone who sees you as inherently untrustworthy? No partner should ever call you a liar without cause or make you feel less-than, judged, or defensive. So tell him he needs to stop that, immediately.