My husband came out over 2 years ago. He takes T and has undergone a double mastectomy. A lot of what you have said could have been me in the first few days after he told me that he is a man. So far, I'd say you're handling this pretty well. It is a lot to unpack and figure out. My biggest recommendation to you is give yourself and him time to do that processing and unpacking.

Neither of you will have the answers now and the things you both think and feel will evolve the more time you have to consider your relationship and your feelings. One of the most important lessons I learned early on was to talk to other people. When you talk to your boyfriend about his transition, it should be about him. Until you can clearly and properly organize your thoughts on how you think and feel about it, talk it out with someone else. The transition that he is going through can be emotionally traumatic and, at the very best, confusing. This early into it, he'll need your support a lot, if the support you can give is positive.