I entered this relationship monogamously and I fell deeply in love with him before realizing I was poly. we've talked extensively about this and usually it's always along the lines of staying monogamous unless I meet a girl (lmao straight boys) But the thing is im also in love with this other boy. I've been trying to find a way to bring up opening our relationship because i feel like this is borderline cheating but I know he won't go for it. Any advice?
If you "know" he won't go for it, then there's not much advice I can give you. If you're pretty confident that your partner wouldn't be okay with you dating another boy, then there are no magic words you can say, no perfect way to "bring it up" that will change his mind.
Stop worrying about how to influence his choices - the only thing you can control is you. You can act on the information you have. This guy has made it very clear that he wouldn't be okay in a relationship where you are seeing another boy. That doesn't sound like a reality you can change - only something you can respond to.
Do you want to stay with this person? Then it sounds like you'll need to let go of your plans to date this other boy. It is possible to want something and not get it, and in fact a lot of people in monogamous relationships have to actively choose not to act on certain desires. Do you want the freedom to date polyamorously? Then it sounds like you might need to leave your current relationship, because "convince the guy you're seeing to be okay with it" is, by your own admission, not something that will work out.
(It is true that there are plenty of good arguments, explanations, etc. about why your boyfriend's perspective is perhaps not the most accurate or healthy. But I'm not going to harp on about "one penis policies" and the like, because if he's made it clear to you how he feels, your role now is to make your choices based on that information, not try to wheedle and cajole and argue him out of it.)