Of course I would tell my son that he's male, a boy and that cannot be changed. It would be cruel to tell him he could change sex, because it's not possible, and people being told it is regularly end up with horrendous damage to their bodies and lifelong health complications that cannot be put right. This increases the suicide rate after transition.
I would much rather help my son to accept and be happy in his inherently sexed body.
That does NOT mean I would say to him he has to spend his life acting out a gender role! Why can't you see the difference? Gender stereotypes are arbitrary, harmful, restrictive social expectations that make life so much harder for those who don't fit into them. You should be able to accept your sex and also be as masculine or feminine as you like.
It happened to me, I was a tomboy and wished I was a boy. Of course I did - because boys seemed to get to do and be all the fun stuff that I wanted.
But it was the 70s and people were still sane and adults told me that I couldn't be a boy, but I could be a girl who got to climb trees and play with airfix and have short hair if I wanted. So I did.
I passed through wanting to be a boy and realsied it was gender stereotypes that gave me that message and I could kick them to the kerb and live how I liked.
As it happened I wasn't gat but many young people in this situation are. The idea that they can opt out of being the sex they are is deeply ties up with homophobia and it is very often gay young people who end up being sterilised and daaged by this ideology, when they could have just been told they could be who they want to be without denying their sex.
And because I wasn't transed, I could go on to have kids and enjoy having a healthy body as an adult.
Of course that's what I would want for my son.