I had a stepdad, too, I just called him dad.
As I read everyone's responses, I see how it may be a bit too formal for my stepson to call me Mrs. ...
I would rather he call me mom, but his mom refuses to allow him to.
It is entirely up to Johnathan. We made it his choice. He said he'd like to call me Mom or Mrs. Strode.
He said he thinks calling me by my first name is not right, because, in his words, he doesn't call any of his other parents by their first names.
If not, it wasn't really a choice.
Here's my perspective. Formal titles are not generally used around the home, because the home is family and a place to relax and be together in an informal way. That is why "formal rooms" are for visitors and places like kitchens are not formal. We all need formal and informal space in our lives.
I think that in my culture, WASP-Canadian 2007, it would be a clear signal that your home, Mrs. Strode's, was not mine. And I think that is very difficult for kids.
It's also just not the norm. I do not think that is the only reason to change something, but it is something to consider.
Eventually your SS will wonder why your home is so different. If you have a really warm and good relationship where he has felt loved, relaxed, and a part of the family, it will probably end there. If not, I think he will need a therapy fund.