If you can, I would take some time to talk out with your boyfriend what exactly makes him uncomfortable about your current arrangement. Can you make some specific agreements that address his concerns without making you feel claustrophobic? Maybe you two can find the middle ground between mono and poly without either one of you feeling like you are fundamentally compromising who you are. This situation with the new guy you are interested could be a nice testing ground for your newly developed agreements, if you are willing to take it slow and talk it out with your boyfriend as you go. Be honest with your boyfriend about your new interest, and talk over what makes him feel safe versus uncomfortable. Maybe he just has some old mono conditioning to work through. Good luck!
Gay 30-something female that just ended a committed relationship with a mono partner, Alex, and in the midst of NRE with a new lover, K.