I do know about the rule where you add apostrophe plus "s" ('s) after a singular noun for possession, even if the name ends in the letter S. But do fiction editors have a problem with that?
I have two characters in my book where this is involved. One is a supporting character named Thomas, whose name I can't change because I've gotten used to the name for this character. The other is one I refer to by last name, which ends with the letter S, and which I can change since he's a much more minor character.
What do you think?