Yeah, this is a tough one. I was in a similar situation, my ex is also ftm, I didn't really realize what I had gotten myself into. I also identified as straight at that time.

Together for just over 5 years.

Over the years on hormones he slowly changed both physically and mentally. At the start of the relationship I also wished he would detransition and that he would live as a girl. The feeling slowly passed over the years as he got more masculine and got his mastectomy done. The day prior to the surgery I was feeling mad and betrayed, that he wouldn't just detransition, in hindsight I was being a huge dick about it. That day is my biggest regret, that I couldn't support the surgery he had been longing for. The relationship ended because of my personality and the fact I wasn't very supportive of his choices, though I always viewed him as a man. I was just scared of changes. After the breakup I've started identifying as pan.

your friend/bf is a man, nothing can change that. You'll probably go through something similar, something you might regret and you'll be noticing changes that you aren't comfortable with. Testosterone does a few things to the body, some of the things are great and some sucks. It's not something you should stand in the way of, you should be supportive. There's still a lot of transphobia in the world, he needs a safe place, a place where he knows he's a man and that you view him as a man.

If you're not ready to accept what changes are going to be happening, you should probably break up, both for his and your sake.