Sorry, but if your child has a preference of a different pronoun, then yes I would expect you to get on board and respect that for their sake. If you can’t do that, that’s up to you and your relationship with them. If they’ve switched back again, that’s up to them. It’s who they are, not who you want them to be.
So your son is male, and indeed is a child, and knows better than you do about his sex?
And, aged 14, he KNOWS he never wants children, and wants breasts, and doesn't want to grow a beard, and tells you he NEEDS to be put on puberty blockers, and knows better than you and knows they aren't harmful (despite all the evidence showing they are) and that they are the only way he isn't going to commit suicide (despite no evidence there's an increased suicide rate in children who identify as trans).
What about if your son decided he's a dinosaur when he was 4, did he know better then?
What if next year he decides he's, erm, I dunno, a wolf, or a baby, does he know better then?
And when your son also decides that he can definitely make a living by sitting in his bedroom gaming, does he know better then?
Or is it just this one area where adults don't know better?