The only comparable situation I have is my 2 bio daughters and my DH, since they never acquired a step mother - my exH didn't remarry. My daughters have never called my DH "Dad" they call him by his first name, as my SDs call me.

I can see why this upset you, but I don't think it will make your son feel any differently about his step mother and you - so does it really matter what he calls her? It's probably just a convention in their house.

Try not to give your son mixed messages in any case - saying "it's OK and it's fine" when it must be obvious to him that neither of these things is true - is going to be pretty confusing for a 7 year old.

I am sorry you are hurting. "Mom" tends to be seen as our special name... What your child calls YOU. (Or Me.) It can come as a shock that it is a title that can be applied to ANYONE. And worst still, used by YOUR child to address some one else.

I really believe that even though this must hurt you to your very core, to your son, it is a form of address used to call a female care giver. Your son KNOWS who his MOTHER is - that is you: Mom! I am also sure that because there is another minor child in X's house who refers to HIS mother as MOM, because X's SO is the female caregiver to someone with DAD, it may seem normal to call that person MOM, especially if DS has not been told what to call her. I do think you may be reading far to much into who calls whom what... (And yes, it doesn't mean it DOESNT hurt.)

I also do not mean this in a hurtful way, but you must recognise that your own behaviour and statements with regard to this situation is dishonest and ignorant?

By ignorant, I mean you assumed that your ex would not let your DS call X's SO "Mom". This is based on how you feel about the situation and the convention in your own home, not based on anything you previously discussed with your X. It appears as dishonest with your child in that you never actually told him the truth: 1) either he can not call the SO "Mom" because YOU dont want it; and 2) that you are OK with DS calling someone else Mom.

You are sending your child a mixed message and he probably knows something is wrong. Please have the courage to speak to your child honestly, as well as having the courage of your convictions to decide what it is you want (with regards to names / how to address people) and follow through on that by telling your son.

I would also recommend that you speak to your X and you two discuss this issue. You can decide between yourselves what the naming convention will be. Understand that because you may prefer that MOM and DAD be exclusive titles, your X may not feel the same way. Come to an agreement for the sake of your son on this.

I personally am not going to be possessive over my "title" my son(12) uses. He calls his step-father by a nickname we all agreed on. If there was someone else that played the "mom" role in my son's life I doubt it would bother me if he called her that. I am secure enough in my relationship with my son, to know that he knows who his MOTHER is and who his MOM is. He can call me anything else: it doesnt change the nature of the relationship I have with him. I think this needs to be considered especially if Dad's new partner treats your son ok or well. Too many people here can not stand their stepkids. Where I do believe healthy boundaries are important, dont give Dad's new partner reasons to resent you or your son. Your son can have multiple care giver type relationships with people: He will ALWAYS know who his MOM is, based on how YOU treat him.

Hoping I can offer little perspective here.....my SDs are older (now 16 & 18) so they're able to express themselves. They always called me Miss XXX. Earlier this year, they asked if they could call me something. They refer to all non-relative adults as Miss xxx or Mr xxx and they felt closer to me than that. They asked me if they could call me mom (they refer to their own mother as mommy). I said sure. It doesn't take away from or diminish their feelings for their mom - it was just a way to solidify our relationship since I am more than just a family friend. My own kids call their stepmother by her first name. Sometimes, my son will call her MommaXXX. First time I heard him say that, it made me pause (because he calls me momma) but it was only for a moment. He's in his early 20s though and I know my kid loves me so I don't mind at all. Your son knows who #1 mom is.

Meh…I am one that thinks a kid should call their parents mom and dad. Unless real mom and dad are out of the picture.
And my opinions have nothing to do with insecurities or such. I think that being a mom and or dad is a pretty special thing and the name should be for the bios only. And PS…I have heard of A LOT of dads that get livid if the SF is called Dad, it is not just a mom issue here. And dang it, it just gets confusing for kids.

Maybe ask your ex to stop him from calling SM mom. Then tell your DS that yes, you are mom and no one else. Maybe you and he can come up with a special name for SM. (be nice…lol). Or tell him that he and his Dad can come up with a special name for SM.

I would be livid if my kids were calling someone else mom. They can call them anything else but I've got the scars, wider hips, bigger feet, and stress lines in my face that have earned me the sacred title of MOM! I would threaten court if my ex did something like that. Hill to die on in my book.

Your son can call her anything else but mom is for you. The younger sibling will be just fine if older half brother is calling his mother something besides mom because she's not mom. Sounds like your ex and his SO need to have a talk w/the youngest child explaining the dynamics.