The only person you can control in any situation is yourself. If he's being unsafe sexually you can insist that he uses condoms with you. For sure he isn't making good desisions right now. He's wrapped up in NRE. Chances are it will pass and once the shine wears off he'll probably wonder why he ever decided to be with her.
I think since he has a track record of conducting himself well in other poly relationships that he will come around eventually. You need to do whatever it is that you need to do to protect yourself emotionally and physically. I don't know what that is going to look like for you because it's different for everyone. Let him know what your personal boundaries are and why they are in place.
Everything will be ok in the end. If it's not ok it's not the end.