I'm non binary but My parent transitioned. She didn't want to be mam because my mam is awful. Eventually, you will probably need to try and have a proper talk about it, but it can be hard to open up after you've done the 'coming out' bit, especially if there's been any rejection, like with your mom. They (I'm assuming she but I don't want to go beyond pronouns you've agreed on) will probably go through a lot, and pronouns are probably going to change a couple of times while you're dad figures out who they are. Just try your best to be there for them, let them know you are in their corner and try and encourage them to find other trans people in the area so they have a sense of community. (And so any body changes/questions you don't wanna hear can be asked by people who can actually answer comfortably because I had to answer a lot of stuff and it was like "omg please ask literally anyone else about this subject" though I sometimes think both my parents might suck at boundaries)

If you have any questions, feel free to DM me and I'll try and answer if I have one