When you say "coming out" are you talking about his being gay, or transgender?
I used a mass email to come out as a nonbinary partial transmale to my siblings. That way there was no confrontation, no forcing family members to make up their minds on the spot, no awkwardness or shame.
You can briefly and casually explain how you've felt during your life, what you are doing about it, maybe provide a couple of links in case someone wants further information on the subject, and leave it at that.
In my case, nobody even commented or seemed surprised.