He keeps calling me mom...

Ok I'm 16 and NOT a mom. My older friends son is 3 and has a speech problem. He doesn't know how to use sentences or a lot of words. He can say words here and there but people can hardly ever understand him. Today he kept calling me "mama" all day long... I don't know why he's doing this. I keep telling him I'm not mama and that my name is Alex but he still continues to call me mama. I'm not really sure how to handle the situation. His mother is starting to get alittle upset that he's calling me that but whenever she's not around he goes back to calling me mama. I don't know why he's calling me that and how to stop it.

Edit: His mom and I look nothing alike. He is however very attached to me and doesn't want anything to do with his mom when I'm around.