Which pain would be worse right now for you in your life & journey; Loosing him as a friend, or hiding who you are? The being misinformed or transphobia you see all around you practically every day.
Generally speaking, whenever the somewhat touchy subjects of TG or Gay Marriage rarely come up for me I try to do my best to educate. Gay marriage more often then trans by a long shot. It is amazing how many people just have no clue at all that marriage is more of a legal construct in todays society then anything else. Even to things seemingly as simple as hospital visitation rights to see a sick/dying partner when "The immediate family" says NO. I'm 6ft tall, deeper then average voice, balding since 14 so if I do any advocating/educating regarding TG I feel pretty safe they aren't thinking "He's one of those...." Most to all people are taught it is a binary. Heck many of us even believe it is. An Exclusive OR of the gender binary. Always male centric. Dangly Bits == Male for MTF. FTMs don't even enter the thought process. Especially when it comes to Bathroom Bills. The press coverage of which is often the only, usually negative, education people get.
As a friend, you know/feel in your heart he is simply mis-informed, just as about every other person around you. A hater would not even be open to the concept of GCS to 'fix' a problem. Again what about everyone is taught is the fix.
Doing some tactful educating if/when the topic ever comes up again I think is a safe path.
Is he a bit of an Alpha-Male type, which would help explain why you trust him more then others you have come out to?
And... if your plans are to fully come out within a few weeks/months, I think it would be best to drop the T-Bomb now on him, get through the initial difficulties and know you have, or not, an ally and possible advocate rather then dealing with such a big unknown about an important person in your life at a crucial time in your life.