First, thumbs up with accepting the fact that you're a trans guy. It's not always easy with all the social pressure and stigma.
I may be wrong, but I think you don't want to be a man. I think you ARE a man. You're already a man. Just because your ID papers don't say so, or just because people think of you as a woman doesn't mean your gender is not legitimate. You're already a man, that's why you may want to do things about it (like socially and/or medically and/or legally).
If that's not how you feel, I don't want to impose anything on you. But presenting things like that can actually be a more positive way of explaining to your partner what's happening.
The more casual you'll be when telling him, the easier it will be for him to take it. Why ? Because when we present ourselves as ashame, ridiculous, broken, sick, stupid, with doubt etc, people will see that and get that idea too.
You're just a trans man, which me you were assigned a woman instead of a man. You came to realize that (and it took the time it took, because of the lack of information, social pressure and stigma, education, society transphobia etc) and now you're expressing HUGE trust toward your loved one : that's a real sign of trust that he could be honored of :)
It may be a bit difficult for him to deal with all the apprenhensions and misconceptions he could have, but most of the time, I found that partners have common "sort of fears" :
- that you will be someone different -> you won't, only your look will change (if you take hormons) but it'll be rather slow and he'll still recognize you :) Your personnality won't change and isn't it what's he's in love with ?
- that he won't desire your body anymore if it changes -> well, only time will tell you both about it. he may be surprised (sex tend to be far better when you're happy being yourself en enjoying life more !)
- that it will "change" his sexual orientation (and mean he's gay) -> it won't necessarily if he doesn't identify as so : the only thing that will change is the label of your relationship (a gay one) and the way it will be perceived by others... It's not always easy, but it's not you the problem or the one responsible, it's the people who hate, insult, get violent and discriminate gay couples..!
Good luck, and well, if every relationship is a work in progress, yours could be deeply reinforce by that, because it's a journey you'll both indulge in, and you'll learn to know yourselves and each others deeper. And if it doesn't stand, well... relationships end... :/
You can have several relationships in your life. But you just have one life.