I'm going to ramble. I don't know if that will help you. My intent is to try to see if anything resonates with you so it can help you sort yourself out. Ok?
Sounds to me like you want to poly (since you both agreed to this and wanted this before dating) but do not find poly with HIM particularly satisfying because it arouses jealousy
Jealousy is a "flag" emotion to me. Like ..."look deeper in here! Needs not being met!"
What needs do you have that he is not meeting? Need to bond before and after his dates?
Is him seeing Another is like "Crap! If you have it to spend, why aren't you spending it on me first, your ORE person to make sure MY needs are being met well before adding on more people? You are spreading yourself too thin and neglecting me even MORE?"
Or could it be just that you want a different poly open relationship model
than him? You are after polyfidelitous and he is not?
Just because you are both polyamorous does not mean you want the same things. Just because people are both monoamorous does not mean they both want the same things -- "cohabitating, married life partnership" flavor. Even the same person may want different things at different stages of their life. I used to want "independent poly solo" when I was younger. Then I wanted to have kids and things changed. I figure it will change again when kid is adult and takes off.
Or maybe you are ok seeing your Other because it helps supplement what you do not get with him. You are willing to "be ok" with that as the price of admission here.
It is ok to date and explore -- and accept not all of them will be a long haul runner if you are seeking a long haul runner type experience in your polyshipping.
Could that be it here? He's been fun to date, but ultimately not compatible for your long term desire?