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!
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.