You may want to see if he can identify why he is feeling jealous. It could be envy. Maybe he wants a relationship like you have. Or maybe it is insecurity where he is afraid you may leave him. (If this is the case, you can try to reassure him that there is no pressure for you to leave him since polyamory helps remove the idea that a relationship is possession of another.)
Polyamory works based on communication. So if you can get him talking about what he is feeling, you will have a chance to work through it. Just verbalizing it may help him process how he feels as well. He may see that is he applying a double standard to you or he may see the picture better.