When to start celebrating Mother’s Day?

I told my guy that Mimi’s Cafe is doing a Mother’s Day brunch sort of thing. My intent was to see if it appealed to him because his baby mama always expects him to do something for her on Mother’s Day & holidays that you typically get a gift from the child on. So I brought it up and his first remark was “you’re not a mom yet” so then I further explained that I was talking about for his son’s mom since she always expected something. He says oh and goes back to watching the draft. No apology or further conversation so I just drop it.

I don’t want to be offended but I kinda am. I’ve been a mother figure to his son because the baby mama isn’t a good mom at all and I’m a few months away from having our baby boy so I just feel hurt that he would say that I’m not a mom. Am I overreacting?