WWYD if your dad wanted to become a woman

[MrsN1984]

I am beyond confused.
I've known for years but now he wants to have the operation.
I feel like I am going to be fatherless

Very, very long story but looking to talk to someone who's been through this.

He used to be the doting dad but the past ten years have been very difficult.
He's unreliable, self centred & has been very sneaky about the whole trans thing.
I found out 14 years ago by him leaving a box of things in my old room after I'd moved out (my parents were divorced but mum let him store some boxes at our house).
I asked mum about it and she found out just after they got married.
I didn't want to talk about it but I never treated him differently. I thought it was 'bedroom stuff'.
Then last year he told my mum he's seeing doctors to start a permanent transition!
He's never once approached me about the subject apart from sending me an email with a picture of him dressed as a woman. No warning and to my work email address.
That didn't go down very well.
He has to live as a woman for some time before having any sort of operation and he text me Christmas Eve to tell me that he will wait until after my sons first bday party.
That's the only time in 14yrs (and he knew I knew) than he's mentioned it to me.

I have nothing left to give him and I have no idea how to handle no longer having a dad X