Step son calling me mommy even though I keep correcting him

Bethany
So my step son is 2 almost 3 now. I've been there for him his entire life almost. But I've noticed when he comes to my boyfriend and I he keeps calling me Mommy. Now mind you I have corrected him every single time. But it does keep happening over and over. I am currently 7 months pregnant with his brother. My boyfriend and I have talked this over and have came to the conclusion if that's what he chooses to call me then that's okay. But I also feel kind of like a jerk for this to his mother. I just am confused on how to feel about this. Any advice?