Want to support a family member who has GenderDysphoria.

[SwallowingBombs]

I don't know where to start.

At the moment he is coming to terms with a few people knowing, although not close family, who won't make life easy.

He has looked at online support groups but generally comes across quite specific 'looking for 'fun'' sites, and he wants emotional and practical support and advice.
Other adults who he might be able to connect with.

He has children and is lost as to how to deal with something that has apparently been a deeply hidden secret for decades, but his primary priority is protecting his children whom he adores.

Hey, your relative should look for local lgbt services, these will be more the social /dealing wth stuff life throws at you side than the "looking 4 fun" stuff. Most big cities have them, and he's the T part of that. The ones local to me accept people from all over, so it's not a problem if he doesn't actually live in that city.
Good luck, I have an ex who went through that and it's ... interesting times.