Maybe ask him to do some reading on NRE and set some temporary guidelines in the meantime (no texting her during dates with you, for example). Not very deep advice, but that's what I've got. It sounds like he's just caught up and needs to remember your feelings. He will almost certainly come around. Good luck!
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.