I would ask your (step)son what he prefers you call him.
If your step-son is fine w/ being called your son, then continue calling him that. I understand you cannot ask him his opinion...and if he's in bootcamp it's probably not the 1st thing on his mind. I would not change what you've been doing b/c of his birth mother. If she doesn't like it, too bad...sounds like you have been more of a mother than she was. However, if it offends your step-son, then stop immediately. If your husband hasn't brought the issue up to you, then he's probably okay with it too. Feign ignorance with the birth mom & continue doing it, but not so often & not too in-her-face.
Sounds like you love this young man. Call him your son so he does not feel differentiated from your other sons.
Your relationship is with him not his mother. She will be more and more out of your circle as he gets older. Lots of people have two mothers or fathers for that matter.
Our grandmother was my mother's stepmother. To me she was just my grandmother. Recently my cousin who grew up near her told me she always introduced her that way since that is who she was to us and she treated us as treasured and loved grandchildren
Nevermind what his mother said on FB. You raised him although she bore him and raised him too.
Since he's 21, I believe it's his choice as to whether you should refer to him as your son or step-son. But since you said he's in boot camp that makes it difficult to address right now. The next time you speak with him just ask him if he has a preference and leave his bio-mom out of the discussion. He doesn't need to know that she precipitated the question.
As for how to respond to bio-mom... make no mention that you plan to ask him at some point. Make no mention of how you intend to refer to him in the future. I would simply respond with, "Hello Bertha, I'm sorry that your feelings were hurt when I referred to Braaaaaydin as my son. I hope you realize that hurting your feelings was never my intention. I love your son as if he were one of my biological children and it's very easy for me to think of him as a son. I know that first and foremost you're his mother and gave birth to him and I would never try to take your place. I hope you had a wonderful Mother's Day."
With the age he was when you met, and the length of time you have been in each other's lives, he is your son. You love him as your son. It doesnt matter who gave birth to him. You are his second mother. I think this sounds like a control issue to me. And I would not give in to her whim on this, especially when she finds it ok to call another person her son, who is really just a family friend! I have five stepkids, my youngest was four when we met, he is now 14 and we have a deep relationship. His mom is a nut and would have likely pulled something like this, but facebook wasnt out then and I would not have given in on it. I give of my self to these children, as if they were my own. They call me mom and I refer to them as my children. At times, I do explain that they I only gave birth to one of our children and my husband brought the rest to our family - when people trip out about me having six kids. On facebook, I have linked my family, and the kids are linked as sons. If she doesnt like it, too bad. legally - I am a parent. And so are you! Oh what fun we have with extended relationships!!
I would tell her your step son has never had a problem with it & that she can just step off.
What does he call you when he introduces you to others? Otherwise, I agree with the other posters who say ask him. He's the one who should get to decide what he feels comfortable with since we always call someone who births us our mother, but many people also feel comfortable acknowledging a parent by marriage as their mom or dad too. I would be really careful to also let him know that you are absolutely okay with anything he wants, including him asking you to call him your step-son if he needs to because it will make it easier with his mother or even just because that's what he wants. I assume it goes without saying that you have his best interest at heart and therefore I would guess would put aside any personal feelings you have about it to honor what he is comfortable with.
sorry i haven't read the responses but there is an option on FB where you can opt not to show certain posts to certain people. i think you have every right to call him your son. aside from step son, is there another way to say it? kind of how there is "birth mom" vs. mom. sorry i can't help more.