As your child is so young OP have you been able to talk about what you plan to do there as you co-parent going forward (whether you remain married or not).
Is you child going to refer to his or her father as Daddy or is there an expectation from your partner to change that? What would it change to?
As others have suggested I would read more of the trans widows threads, stories and resources. This is a very specific situation, as a PP says, to prepare for a tricky time ahead, but please prioritise yourself and your child. You are not responsible for anyone else's happiness.
If surgery was on the cards - actually even if it's not - I would personally be thinking about whether I could maintain a healthy sex life (healthy as in mentally, emotionally and physically) with the man I was in a relationship with, if he intends to dress as the opposite sex. Do you think you could engage in sexual with him if he was imagining himself as a woman having sex? If he intends to invert his penis then that is even harder to navigate. Could you be in a celibate marriage? Could you have an open marriage? It's possible, couples do it, but it might not be what you imagined for your future.
Some of the sexual experiences of trans widows are very harrowing to read. It might not be possible for you to be happy in this relationship going forward and if that's the case it's better to be honest about that, early on, rather than drag it out IYSWIM.
This is so very difficult, it's a lot to deal with, I can only imagine it's a type of grief - I've always thought that's why the partners of men 'transitioning' are called trans widows.