You know him best and if you think this is gonna break his heart it probably will. Same thing happened to me.
There's no right/best way to do it if the outcome will be the same. But I can tell you the stages I saw:
1) Initial friendly curiousness and disbelief/not taking it too seriously
2) Realization you are serious and then he gets serious because he starts thinking of the implications
3) Grief and displeasure that you didn't say before he got involved
4) Difficulty thinking of you as male and/or referring to you as male, not wanting to overtly acknowledge it
5) Making decisions about the future, whatever that will be, in my case we are still friends and still maintain a close-ish kind of relationship, and still live together, do not know the ultimate future though
That is from a 10 year relationship, I am not sure at what stage of trust and confidence you are in with your bf. Mine accepted that transition is a given, it has to be done, and since I helped him in the past he feels he owes me support in this. When I told him the first time, it was a casual conversation, maybe even a fun sort of conversation, and it went well. However, once it is obvious you're going to act on the situation, that is when you will encounter resistance. How quickly it gets to that stage might depend on the nature of your relationship.
