Step parent boundaries
Hello! I'm not sure exactly how this works, so I'll just shoot. I come here because I need advice on a situation that not many people can give me, given that they have never been in this situation. This seems like a decent place to try and seek some kind of answer. I've been in a relationship for about 9-10 months now with someone whom I previously dated a few years back, before he had a child. We now live together, and his son is just about 2 years old. The child's mom and I get along well, and do not have any issues whatsoever. The only issue I have is that he, at times, calls me "mommy" to his child. He will say things like "go get mommy," or call me "mommy _____," with my name at the end. This makes me extremely uncomfortable. I have voiced to him that I do not like it, and that I do not feel it is correct to have his son say such things because it may lead to confusion as he gets older. As the months have gone by it just keeps creeping in, and I hear myself start getting called mom again. The paternal grandmother even made a remark when the child called me "mama" and I very nicely told him "my name is _____, I'm not mama but I care for you very much," which caused her to almost be irritated with me. Almost as if I had done wrong by telling this child very nicely that I am not his mother. The child's mother is in his life, her and the father have 50/50 custody. The father and his side of the family do not like her, she is not the greatest mother but she still is his mother and I don't feel like it is my fault that this was who he had a child with, she also would absolutely not be okay with her son calling another woman mom. In a way, I feel wrong for it but I am not sure if that's just because of how he presses it on me or if I actually do feel wrong. I do love the child, don't get me wrong. It is just a strange situation that I can't seem to communicate through, since it will just go away and come back a few weeks later. Seeking advice, any advice. I just am not sure where to turn for this.
Edit: I also want to add, on the weeks that his child is with us it is like I have to take on a role as if I am this child's mom. I know dating someone with a child is different, but is that inappropriately different? I change diapers, I feed him, I watch him, I wake him up and dress him if dad just can't seem to get out of bed with enough time before work, make his lunch for school the next day most days, if dad has to work and daycare is closed I have to find a way to make it work with my work from home schedule. I just feel like a lot of weight is put on me for someone so young, and someone who has never had plans on having children of their own.