I'm very sorry you're going through this.
I know you said he discussed it before you had your DS, but I think you said, not before you were married?
If that's correct I'd feel very angry that my choice to be in a relationship with a man who was happy being a man had been a lie.
It's hard when you love someone, to just detach from them.
I have no personal experience of your situation, but I have read a lot of accounts from people who have/have wanted to transition 'have always wanted to be a girl' and the ones I have read have had done life experience/trauma that hadn't been addressed. Therefore I'd be making sure DH had had done quality therapy, for his sake & mine.
Not to change his mind as such, but to make sure that transitioning is the solution to how he feels (so often it appears it's not).
I wouldn't make any decisions until that had happened.
If at the end of that he decides to transition, I'd end the marriage, support him as a friend and live my own life which doesn't include having sex with a trans woman.
As for your DS I'd say it's better when he's as young as possible. Kids these days are so much more accepting, try not to worry x