Sometimes it's hard to be patient. I don't really know where your boyfriend is, so I can't really speak directly to what he's feeling. But I think a lot of people just benefit from the feeling of "moving forward" and are afraid of stagnating in the status quo. This is especially true for trans individuals, since a part of their "status quo" is deeply troubling to them.
If you're thinking that's not necessarily a good thing, you're right. Sometimes people like that can move forward without fully considering things. But sometimes it's fine. And sometimes, even if things go a little wrong, it's not the end of the world. As has been mentioned, it will be a bit of a sudden change to see his head shaved. But it'll grow back. He'll find some hair length he's comfortable with. And I think once you see him comfortable, you'll feel better about it too.
Regarding your feelings, your fear is entirely justified. Don't feel bad about it. It has nothing to do with supporting him or not supporting him. What you're going through is hard.
For the trans individual, although they recognize it as a dramatic change, they think of transition in their mind as just moving closer to who they really are. They see it less as becoming someone different and more as becoming _themselves_. But in terms of their outward presentation to the world, in terms of what you see, they are becoming someone different.
This is a very complicated journey of change and discovery. Depending on how far down this road they decide to go, your partner may be changing a lot, probably more in the next few years than most people do throughout their entire adult lives. Everyone is afraid and uncertain about this process, trans people themselves included.
And everyone goes through a sense of loss. Even many trans individuals go through a stage of grief where they struggle to deal with all the parts of themselves they've left behind. The people who love them do too.
It's really OK to feel fear or loss or uncertainty, or any number of other things. Your partner will probably feel all of those things too. Just don't let those feelings rule you, and don't doubt yourself because of them. Yeah, a lot will change, and sometimes there will be tough feelings to deal with. But if you love them, and want to be with them, and you're happy with them, that's all you really need. With love, and concern, and compassion, and it sounds like you have all of these for your partner, the details can be worked through.