It sounds like you need to have a talk about your boundaries and what you need to feel special and important. It sounds to me like this is all going very quickly and that you're not able to keep up emotionally. It's ok to tell them to slow down. It's also ok to tell him what you need.
I'm not saying that you are cut out for poly for sure but if you love him it's worth giving him a chance. Push back the wedding date if you have to. Is he willing to talk things through with you or does he just do what he wants to and let you just sweat about it? Not calling you when he said he would is disrespectful. You deserve to be treated with respect in any relationship that you're in.
Expect some heavy conversations when he gets back and a lot of tears. But I want you to know that what you are feeling is normal. Poly is a lot of work for everyone involved. If anyone isn't willing to put that work in, on getting to know themselves and getting to know each other, poly probably isn't for them. Good luck.
Everything will be ok in the end. If it's not ok it's not the end.