For the most part, jealousy is treated more as a sympton that an issue. The issue is usually insecurity or envy. In your case, it sounds like insecurity because you will see it as competition.
Maybe you can perceive it differently if you realize that in this "competition" everyone can be a winner. Unlike monogamy practices, if you like someone (for whatever reason) you can be with them and do not have to choose one person over another. The only reason you leave someone is because you just don't want to be around them, not because there is someone better out there.
In a sense, love between any two people is very unique. If he was involved with another woman, his love for that woman would feel different than his love for you (not better or worse, just different).
Another poly concept is compersion (or frubble). Sometimes seeing your partner happy being with someone can make you happy.
But it has to work on communication and understanding each other desires.