My "stepson" would like to call me mom is this ok? Advice:?

I have a 12 yr old stepson. I'm currently 9 months pregnant. His biological mother is not very involved in his life and the child live with my husband & me. He sees his mom about 2x a yr. I have been in his life for 3 yrs now. I made it clear from the start my role in his life as a parent & that I was not there to replace his mother. Yes I do parent, I provide limits, we share time, play games, I cook, clean, and care for him when he is sick. I help with school work, attend school conf.'s so I am raising him & expect him to respect me. I have never imposed him calling me by the title mother. he uses my given name. This is ok with me but he asked if he can call me mom. i wonder if this is stemming from my pregnancy. I never call him my stepson I ALWAYS refer to him as my son everywhere we go so he doesnt feel segregated. While his mother isnt involved I wonder if this could create tensions. I dont want to reject the child. I want to do what is right & responsible w/out hurting the child