It is important that you feel like you are safe in a relationship, and it is also important to try to go out of your comfort zone a little too. Comfort zone and safe zones don't have to be the same thing. My mom doesn't like camping, but she still does it to hang out with my dad, my dad likes classic rock, and my mom hates music with very much of a beat, so he doesn't listen to it much around her. Relationships are a place of give and take.
This is my opinion on the matter. You shouldn't feel like you have to be his mommy, that is a very involved role, and I imagine it can be mentally confusing. If you felt like you have had a good day, or he has been very kind to you, it might be nice of you to try out some care-taking roles, like bottle feeding him, or just cuddling him when he is diapered. That obviously though is out of the comfort zone and really shouldn't be expected.
As for how I think it would be good of you to give to him, would be acceptance on the level, that you are willing to let him wear diapers when he feels that it is necessary for him due to stress, or its been a while, so long that it isn't constant and interfering with your and his intimacy and interaction. It would probably be a really kind way to show acceptance by allowing him to wear an exposed diaper around you, regardless that you will not be interacting with him, however, I think at the least, he will eventually come to feel the need to have a safe zone, even if on his own, that he can be freely diapered, even if it has to be while you are out at the store or something.
Here is the fact of the matter. I know how it feels to think to myself, "maybe I could have the strength to stop wearing diapers for this girl, I love her, I want her to be happy," however, it isn't really being honest with myself. I, and most Adult Babies, and probably him, are naturally this way, it isn't a hobby that we just picked up, it is something that most of us were born with, or wired to think. Asking him to give it up entirely, would be like asking Da Vinci to never paint or make any form of art, he might have been an interesting guy still, but it would take away a key aspect of his personality.
Have you seen inside out? If not you should really see it. Imagine the Hockey island, as it drops into the abyss, that would be kind of what would be happening if he just ups and quits, it is kind of a key part of his personality, whether you see it immediately or not. some of his core sensitivities and kindness is probably tied back to his desire to be little.
Anyway, best of luck at sorting things out. I hope something that I had to say was helpful.