Ok, first off how can you, as a bisexual woman, call his interest in men "a weird phase"??? That's just so dismissive, I don't even get it.
That said. What was the boundary he transgressed? And when he slept with his new guy, did he have your ok on that?
It is perfectly reasonable to ask to meet and get to know his new bf, ideally before they get more serious. Is he open to a coffee date for the three of you?
It is also perfectly reasonable to ask him to not go off the deep end into NRE by devoting a full week to the new guy while you need his help. In fact, that's pretty thoughtless of him. There Is nothing wrong with taking it slow and respecting your existing relationship(s) even as you're establishing a new one. He "can" do whatever he wants but that doesn't mean he should if he cares about your feelings. Tell him how you feel, tell him you think this is too much too fast, and ask him to slow down. If he won't, ask him why the heck not.
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.