I grew up with a step-mom and I myself am a step-mom now. My step-son is 10 but I have been in his life since he was 3. He asked me if he could call me mommy and i told him he could if that was what he wanted, but his mother found out and did not like it so he calls me by my first name, but when he is at our house he will call me mommy sometimes because he doesn't have to worry about his mom finding out.
I don't think you will confuse him. He will always know who his real mother is. As far as the confusion about going to mommies house we always told my step-son we were going to his other house and he understood that. (I think that also helps him to feel that our house is as much his home as his mother's is).
I think you should sit down with him and your fiance and discuss some of the options that you have and let your step-son decide so you are sure he is confortable with it. Hope this helps and good luck!