Welcome to the forum and congratulations on your relationships. So many say that poly relationships are inherently unstable, so it's great to see more examples of folks that are making it work.
Jealousy can definitely play a part - what work have you done (as a group) to identify what specifically is causing this? Usually jealousy is a sign of some need not being met, while they perceive that another (you) is getting what they need. You say it is usually about the intensity of the relationship - what aspects of the intensity bother him? is it the lovey-dovey stuff, or the intensity of the sex? How would you characterise it?
"Listen, or your tongue will make you deaf." - Native American Proverb