First, you misunderstand. I'm not suggesting trans people think their way into being the opposite sex, I am saying if you believe they genuinely are the opposite sex mind in the wrong body, you are saying the minds of the sexes are meaningfully different. You are accepting that some ways of thinking and feeling are right for a man or boy and others are right for a woman or girl. You are accepting boys and girls exist abd are different, but using the sexist construct of mental gender over the simple one of physical sex.
Secondly. So what is this FEELING?
Why does FEELING like your body ahould have been female override the reality that it is in fact male?
I'm 5'10. If I FEEL shorter than average, does that make me interchangeable with someone who is 5'1? Will her trousers fit my legs because I FEEL they should?
Can you see the problem here? You haven't really explained at all why your boy is actually a girl, or has anything in common with girls at all. You've just decided his FEELING that he is like them matters more than their reality that he is not.
Another question for you. Is a trans "girl" actually a girl - which unavoidably means you do accept there is a meaningful, defining difference in how "boys" think and how "girls" think, or is a trans girl a boy who believes he is a girl, so to be kind you are happy to pretend that is true?