I wouldn't bother spending any bandwidth on trying to figure out if his gender transition is legitimate or not. Either way, switching genders does not give you a ticket to become an irresponsible parent.
An acquaintance of mine underwent a gender transition. I attended her wedding when she identified as male. She had pursued her now ex-wife for a fricking YEAR before ex-wife said yes to dating her. Like Lexie said, many trans people go through extreme efforts to extinguish the voice that tells them that action is required.
Concentrate on your daughter, and you will be good.