A few things stand out to me.
First of all, good for you for reaching out here and venting it all out to lift the burden a little. I hope that helped.
Next, you seem to have a classic issue that many have. Some are okay with their partner having sex with another, but no emotions, while some are okay with the emotions, but not the sex. You're okay with him having one or the other, but both together seems like too much.
But here is what could help. You say you are really close and tell each other EVERYTHING. I suggest it's not really healthy to know everything about your partner's feelings for and activities with their other partner or dating-potential person. Hubs is in NRE (new relationship energy), infatuated, where one tends to be very excited and even obsessive about a new person. This doesn't last forever, but it needs to be managed while it's happening.
My current male partner was new to poly when we started dating (3 years ago) and we've had to work out a balance of how much he should tell me about other people he's been dating. A little bit is good, too much is TMI. It can make me uncomfortable, and it also violates the other person's privacy.
So maybe just asking your hubs to chill and not constantly bring up his new friend would help. Maybe he could find another poly friend to talk to, or just journal it, or something. I don't want my bf to constantly throw his new dating partners in my face. I want our time to be ours. I am not his bestie or his mom.