I'm the parent of a transgender woman, so maybe I speak with a little authority. It's not what you want to hear, but you may not be at the stage where you should write this. It's pretty clear from your question that you don't know any trans people well enough to ask. If you don't know trans people, how likely are you to portray one you made up with full plausibility?
Setting that issue aside, you say you're in third person but you don't say whose point of view you're in. If it's someone other than the FtM trans man, then when this person is still presenting as a female, she would be referred to as she by any POV character. They don't know she's about to transition.
Once she makes the transition, a caring and think-before-you-speak POV character will use "he" consistently--but if they knew him for long before he transitioned, even a kind POV character will slip on pronouns (and hurt or anger the trans man) many times.
· 4 weeks ago