"I think I would be okay with him exploring too if I'm part of the exploration."
You may wanna consider the fact that he and one or more of these women may develop feelings for each other, and you then being present/involved/a part of things/whatever may not be such a feasible option any more. These sorts of things can't necessarily be controlled. I would strongly recommend doing whatever you can to get a handle on your jealousy before jumping into new things.
"He knew they would be disappointed in him if he went ahead anyway."
Just wanna reiterate that "I'll keep this important info a secret so as not to upset anyone" is a really, really bad trait in a poly partner. Here's hoping that his tendency to lie rather than show integrity doesn't come back to bite you...
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.