Well...now that all that is out of the way, I'll share my experience being a step-mother.
I married my husband three years ago, and it took two years for my step-son to even mumble the words "step-mom". I think a lot of it had to do with the fact that, given the situation, he thought that if he called me "mom" in any fashion that I would replace his mother. From my perspective, that wasn't the case at all, so I didn't push it. I was "Michele" to him and "his Dad's wife" to anyone who inquired, and that was fine with me and I believe that it reassurred him that I wasn't there to replace his mother.
See, when you're four (which my step-son was when I came into his life) or five and you're thrust in the middle of their divorce, calling anyone "mom", no matter if there's a "step" in front of it or a name behind it is probably threatening, and to push the issue on him right now might be more of a detriment to your relationship later on down the line. Let him start to trust the situation he's in, and to trust the relationship between you two. Give him your reassurance that you aren't there to replace his "real" mom.
I can guarentee you that he's not asking you to do this because he hates you, or is trying some kind of emotional blackmail. He's just letting you know that he's uncertain about the relationship between his mother and himself, and he wants to make sure that she stays an important figure in his life. He's already lost a lot of time with her, and he's probably scared that he's going to lose her entirely. You have to remember that your step-son is in an entirely new, scary, confusing situation. He doesn't understand why his mother isn't living with him all the time any longer, or why he isn't seeing his dad the way he did before.
Once he realizes that you aren't there to REPLACE his mother, but you're there to provide ADDITIONAL emotional, mental and physical support, he'll trust your position in his life, and might be able to come to accept the fact that he has a "bonus" mom ;)
Believe me, it's worth the wait. It took two long years, but to hear my step-son voluntarily call me his step-mom when people ask is pretty special to me.