What you're feeling sounds very normal. Jealousy is basically the feelings related to the fear of losing something you have to someone else. You don't feel jealousy in relation to your more established relationship because it is much more secure simply by existing longer. Your other relationship is less secure because it has not been there as long, plus you mention that he isn't big on the poly thing so the danger in your mind that he will leave you for a mono situation is more likely.
The biggest thing to think about is that those are your feelings, not his. They are your responsibility to take care of, if that means you just need to mentally reassure yourself, or that you need to bring them up and talk to him about them, is up to you. But your feelings are yours. One of the problems I see (mostly my mono friends) make is making their feelings someone else's problem or responsibility. Taking out their feelings of jealousy on the object of those feelings without dealing with why they feel that way. Good luck with your situation and God bless.