See, he's right. Nobody can change their gender.
What he doesn't realize, though, is that we're not changing our gender, we're embracing it. It's not like we magically woke up one morning and said "Yeah, being a man was great and all, but I think I want to be a woman now." I don't ever recall a time where I consciously decided that my mind should be programmed to "expect" my body to be female and thus feel dysphoria. The last time I checked, my body wasn't programmed to run on testosterone ever since I was born, so I haven't changed anything, I'm just embracing who I was born as.
(I know that's not what he meant, he's a bigoted a**hole, but my point stands.

)
Hell, my mom even tells me, she didn't recognize the "male" self that emerged when I went through puberty. But now, she "finally has me back," and once again sees me as the same person as my childhood self again. So again, I really feel like I didn't "change," I didn't "transition," I just corrected a hormonal imbalance that was trying to turn me into someone that I never was in the first place.