You do fill a maternal role for your "step son". He is close to you, and you to him. You love each other, spend time, and you are responsible for his well being. It's wonderful that you, his mom, and your partner have his best interest in mind, and are working TOGETHER!! Yay, you guys!!!
Why not ask him if there is a special name he would like to call you. For example, I am Mama / Mama Ephie to my bio niece, and my sister (her bio mom) is Mommy Rita. Some families have a Daddy, and a Papa. My husband is Uncle Max / Daddy Man / Daddy Oyster (my niece came up with those). You could also have an endearing twist on your name, that's just for him. For example, if your name is Alisha, perhaps he would like to call you Li-Li, or some other name that is endearing, familial, and gives voice and recognition to your special role in his life. Also, there are different names for mother, in different languages. Perhaps one would fit for you? We have THREE sets of Grandparents for our children (actually five, but one is estranged and one passed on). Each has their own name and is important and valuable. We have a Nana (my mom), a GeeGee (my step mom), and a Grandma G (my MIL).
We can never have too many people who Love us!
You could leave the communication door open and say something like, "Mommy is the name you call your mommy ______ (insert her name here). Is there a name we could use for me that would be special like mommy is for you and your mommy, but that's for us? I'm so glad I get to be one of your mommies, and that I get to have you in my life! I'm also really glad you have your mommy. We all love you!"