Maybe you could try telling him something like, "This is hard for me. I have to work to be able to share you. When you downplay it and make it seem like your other relationships aren't anything special, you make me wonder **why is he doing this to me if it isn't going to enrich his life???**"
I had a similar talk with Easy. I told him, "This hurts. It isn't easy. I don't like changing and I don't like sharing. I'm doing it because I love you and I love Asha and I want you both to be happy. So if you don't love her and have a good time with her, if she doesn't make you happy when you're with her, why am I putting myself through the ringer? It makes me feel worse when you aren't happy than it does when you are. Just be honest with me. Share your happiness so that I can see that all of this is worth it."
I think it helped him. He seemed to stop downplaying it after that, and tried instead to find a way to be happy *and* show that he was happy to be with me at the same time.