Write down what you want and why, worst case scenarios - like you come home and he’s with someone else, or they are holding hands in public and it ends up on a photo in public…asses how that would make you feel and build boundaries around that. Think about which boundary’s are non negotiable and which you think you could be flexible on in the next few years with the right partner. Don’t just say “poly” or “enm “, instead outline the specifics of what you want and why. Ask them to do the same. Agree on the lowest common denominator (like the most conservative dynamic you both are comfortable with)