I would be supportive of my child's feelings. I would tell my child that these feelings are sometimes temporary, especially in the young, but sometimes they last a lifetime. Clearly a person of an age that can be described as "child" is not ready to make a permanent decision based on feelings that may be temporary.
I would be non-judgmental, and explain the upsides and downsides of any choices that they are contemplating. I would particularly explain that there are adults who predate on children who express gay or trans feelings, just as there are who predate on heterosexual children, and so to let me know immediately if any adult wants to discuss sexuality with them. I would also make sure they understand that while passing of the AIDS virus through heterosexual intercourse is pretty rare in the United States, if he chooses to have sex with a male while playing a female role, that would be homosexual intercourse, not heterosexual, as even a poster like TheProgressivePatriot must surely agree.*
I would be very leery if an adult at school were influencing these choices in any way. I would visit the principal and insist that these conversations be left to parents and other family.
*I would request that you affirm that you agree with that so that I can show this post to any grandchildren who express that they are trans. As you know, there is some element of breaking with reality in the transgender movement, so I would want to make sure that is not taken too far.