I would suggest just to talk. If you feel honest jealousy, take a moment, figure out what the reason for this jealousy is. It's very helpful to identify the specific triggering factor for the jealousy.
Then talk to him and also eventually his other partner. I believe that "don't ask, don't tell" communication relationships are very hard. If you want to work through the jealousy, I would personally need to talk it through with not only my partner but also with the person that I was experiencing the jealousy of him with.
If you can't overcome the jealousy though, it might be better to just take a step back, deep breaths, clear your head and then once the urgency of your feelings passes take a look at why you had those feelings. If you can't. I would personally take another look at whether this relationship is worth pursuing and if you are capable of overcoming the jealousy. Sometimes poly relationships don't always work. If you go over it not just by yourself but with your partner, you may come to find that you are hardwired to be monogamous. And that is something that would be something you'd have to figure out. Rather than trying to push for something you aren't able to do, realize that you aren't made that way and fix it.
Just something I wanted to insert. I hope you can get over that jealousy. Good luck.
Female - 24 - poly