You tenderly reassure him that you love him as though he were your own, but that it would not be right to tell people that you are not his step mom because that would be lieing and lieing is wrong. Tell him that you understand how he feels but that you want to set a good example for him in honesty.
My step daughter introduces me all the time as, "This is my mom." I don't correct her in public because that might embarrass her like a public scolding. But she has lived with me and her dad for the last 8 years and doesn't see her
BM that often. So to her, I am her everyday "mom" but she also knows who her biological mother really is. She calls her "mom" as well. She and I are very comfortable with that, though her mother is not. But that is her mother's problem, not ours.
Perhaps you and he could think of a better term that he is more confortable with! Something that will make it special for just the two of you! Something to the effect of "Yes, I am William's Bonus Mom!" But to say, "I am not William's step mom, I'm his mother," would be a lie and would set an example that lieing is ok.