So is the first person that he has fallen in love with? Is that the issue here? The rest were all super-casual, and obviously transitory. This one, if he is truly in love with her, may not be. Maybe that is why you feel threatened by her.
Polyamory is about falling in love with many people in an open and honest way. It seems to me like the only reason you have a problem with this is that it's working rather too well for him...
"Listen, or your tongue will make you deaf." - Native American Proverb