Hello,
I think a good first step is beginning to call them by their preferred pronouns. They're going to need all the support they can get through this hard time, and if they prefer you calling them your boyfriend and referring to them as "he", it'd be good to begin doing so now. It's hard enough getting misgendered out in the world, but if you're there with the support, it makes all the difference in the world.
The first thing they should probably do is seek out a gender therapist. That's what I had to do. They should also do some research on top surgery/attaining T in your state. A site I find helpful is
theartoftransliness.com. It has a lot of resources if you need.
I'm not "fully out" to my parents, but there's lots of pamphlets and such things they can probably give to their parents. They're going to face a LOT of problems - like using the bathroom, for example; there's a list of things on the site. I think browsing that site will help both of you.