I see your tags indicate jealousy and cheating.
I'm assuming this his baby? If so he needs to be helping you and supporting you instead of putting his wants first. If not then he needs to show his commitment to you by being honest instead of hurting you. If he is wonderful, he really isn't showing it.
The relationship needs to be strong regardless of whether it is open or not and if you're making the good faith effort to not lock him out and be poly, he needs to return the favor and be honest.
Note neither you nor I have suggested he can't be involved with other women, just that he not lie to you.