I don't that's actually needed? If it's not a hard focus, I don't believe anyone will care, it's not like it changes a lot - unless it does change a lot, which is when you should tag it, I guess. I believe that it depends on how much the reader has to go with the gender bender idea, I think?
For example, in one of my stories, there's a dude who can his physical structure, just not his innate features - get older, younger, male, female, more or less muscular, but will never change his inherent face or height, it will simply adjust to the other parameters (age, muscle, gender). It's important to the character, yes, but not so much to the story. He's a side character. He changes again and again, but that doesn't affect the story in a deeper sense. As a reader, I think one could hardly care less if a scene us about a dude described as a woman and sometimes as a man, depending on the situation.
The question is: In what context does your character change gender, what does he after he changed, how much is the reader involved in his mental state while he's bend, and how long does it last?
