I don't think it makes sense to tell him he *shouldn't* be jealous, or that he has no reason to be jealous, because that's not true. Of course he should be jealous, because he
is jealous, and of course he has a reason to be jealous, even if it's one you don't think is valid.
I do think it works well to be as loving and generous as you can with him, to reassure him that you have chosen him and you continue to choose him, and that you are totally committed to being his LLTMBR partner, if indeed you are. If you're not, now's a good time to explore 100% responsibility and see who you're being that you find yourself in this boat. Here's your chance to expand into your most loving and generous self -- I think that's what jealousy is here for.
And as Ruby mentioned, ultimately his jealousy is his; it's not your job to fix it for him. Kind of a dichotomy, huh?