I cant give you much advice, just that I can just imagine how truly difficult your situation must be.
I, very recently unfortunately lost my long time girlfriend. I met her stealth, we fell in love, and she was accepting and the most amazing person ever. With her, I literally have the best memories with. Im still in love with her, but she left me for someone else, then, she messed things up even more when we tried to work through things, and now she is happy having her freedom to act single. So, though my situation is different, I can empathize with what you may go through, of losing your loved one. It is especially difficult for you, as you are choosing between living your life as you, or living your life with the one you love.
But if you are 100% sure transitioning is what you need to do for you, then you have to do it, and like you said, the doing it is whats so hard. Its like, 90% of you could be logic, telling you what you need to do, but that 10% that is emotion, just overrides the logic. But I guess you do need to go for it, for you.
All I can suggest, is to try your best to keep him and his family involved in what is going on. Communicating with them what is happening, and why, would be your best bet at keeping them in your life (in my opinion). They may struggle at first, they may struggle immensely, but you never know, they could stick out the whole transition by your side, and your partner may be able to go through it with you and still want to be with you. There could be a chance. But dont put your hopes in this. You have to transition for you, make sure you dont do it in a way that is for you and them, in a way where, if they turn their back on you, you fall apart without them.
In terms of my own family. The first few years of my transition, and they were some of the worst years of my life, they were not "for" my transition in any sense of the word. I wasnt allowed to even talk about it with friends. I had to move out at 18 (I know this age is fine for many people to move out, but for me, in terms of feeling ready to face the world, I was not ready! Pre everything and jobless :/ ). It sucked because me and my family were so close, they were all I had growing up. But, though it truly looked like Id be on my own, I can now say Im incredibly close to them again, and the relationship with my Mum has changed in a way where I can truly say she sees and treats me as her son. Finally. Im just telling you this to show you that people can accept things, even if by doing so in their own time means it takes years, itll take years but it can happen. So, if you lose your boyfriend now, he may come round to everything eventually. But in the meantime, you will have to learn to really be strong for yourself. I hope all goes well whatever path you take.