My boyfriend wants to have a poly relationship and I've expressed how uncomfortable it would make me. He says he wants me to be comfortable and trusting enough to do it. I'm afraid that if we end up trying it, that I won't like it and he'll continue and I'll be uncomfortable.
When you say that he wants you to be comfortable enough to do it, I'm not sure if you mean he's said "I want to do this, but only if you're comfortable with it" or "I want you to become comfortable with this."
The first one is fine; he's told you what he wants, but seems to understand that you don't want that. If you're not able or willing to try polyamory, that's totally your right. He can decide to stay in a mono relationship with you, or decide he has to leave the relationship because being able to pursue polyamory is a dealbreaker for him.
The second one is less fine. You don't get to just ask someone to have a feeling; we don't have little knobs inside our brain we can fiddle with and get ourselves to the settings that someone wants us to have. If he is trying to pressure or guilt you into "being comfortable" with something you simply aren't comfortable with, that's not okay. Tell him to drop the issue, and if he won't, leave him.
Your last sentence also concerns me. You're dating someone who, somehow, through his actions or words, has made you worry that he'd continue doing something even if you're uncomfortable with it. It has nothing to do with polyamory - if my boyfriend invited me to try rock climbing with him, but I was worried that if I tried it and didn't like it, he'd continue to pressure me into doing it, not accept my expression that I didn't want to anymore, and try to hold me permanently to my initial willingness to try it out, that would be a major red flag that he just isn't healthy to date.
It's completely fine to try something in good faith, then realize, based on what you learned while trying it, that you don't like it. If someone won't accept that, or bulldozes over your feelings, they are not someone you should be dating. Do not be with someone who makes you worry that they won't listen to or respect how you feel.