I have DID (aka "multiple personality disorder")...and fortunately, people *do* take me seriously.
One of the things abusers (and their enablers) use to control children is "You're just a kid, no one believes you".
In my case, I'm not delusional (DID people aren't; people who are schizophrenic, otoh, are)...and we (my therapist, also my dr) believe "Beth" is my primary "self." This is based not only on what I experience when I'm "her" (peace, serenity...and being clean (as opposed to being "filthy" when a man)), but also because the "others" inside are at ease with her, but with none of the others.
In fact, in the new SOC v7, DID is mentioned as resolving (or its effects mitigated) during TG transition in some cases.
This person claims that one can't change their gender...which is true, as far as it goes. We were all WOMEN at birth, but we didn't recognize it until a certain point in our lives (everyone is different as to "when"). I don't think anyone here was genuinely a "man", then one day decided, "Hey, I think I'll just grow some boobs and be a woman." There are far too many hurdles for that too happen (except maybe in the porn industry, or with people who are...odd.)
For him, changing genders was probably not a good thing.