First off, please refrain from using all caps. It is often construed as "shouting" and is quite difficult to read.
Second, I am not entirely sure you read my post all the way through, or if you did, you picked a few sentences out of context. The boilerplate in Georgia may be different than it is in FL concerning "shared responsibility," which is why I inserted the caveat about "where I live."
IF you and you ex- are on reasonable terms (which it sounds as though you are, good 4 you, but bear in mind that yours is the exception), and the "other mother" did not carry on as though you were dead, perhaps it is workable. Also, you do not mention either the age of your son at the time of your ex-'s remarriage or how much time elapsed between your ex-'s remarriage and your son's calling her "Mom." I imagine that it did not happen overnight.
In both the situation described in the post to which I was responding and my own, the first condition was not met. Unfortunately, in my situation (which, admittedly, I did not describe blow-by-blow), the (now ex-) stepmother carried on during a 3-month courtship and early on in the new marriage as though I were dead. My daughter was also told half-truths and utter lies about me by my ex-and his family, which the stepmom believed as gospel. Nevertheless, DD was free, e.g., to display/carry in her wallet whatever photos she wished. Ironically enough, now that the ex- is divorced from wife #2, HE has a bigger problem with DD carrying a photo of the ex-stepmom in her wallet than I do. In the situation the poster described, this issue about nomenclature was merely another skirmish in a long-running battle-of-the-exes.
Please re-read my last paragraph...yes, it is all about the child, which is why I concluded that the relationship w/the stepdaughter needed to take precedence over the title.