I'm wondering how long it's been since you opened your marriage. I'm also wondering if he's doing any work at looking into why he's insecure about other men being involved with you. These things can take time and work but he has to be willing to do the self reflection to figure out what is going on for him so that he can move past it. It's a pretty common place for people new to poly to start as it feels safe. The thing is you're not happy with the structure. If he's not willing to do the work then no amount of resoning is going to change his mind.
I also have to agree with other posters that he has absolutely no say in whether he becomes sexually involved with any partners you happen to have. If there's an attraction there, great, then you can all be involved together. If not, just because she's dating you doesn't give him any right to her body. Personally that would be a deal breaker for me and there's no way I would get involved with someone with whom the spouse was a package deal.
Everything will be ok in the end. If it's not ok it's not the end.