Maybe best is to just continue using their preferred pronouns as if nothing happened, and be kind to everyone. That will already show your support and he might come to you if there is anything to talk about.

I hope all the other colleagues are supportive as well, I would have a close eye on everyone now.

Boss called me in because his computer was slow. What I saw was a desktop filled to the last pixel with files, stacked on top of each other and hundreds of open browser windows with hundreds of tabs each and a blurry background image of the building we were in (honestly a beautiful old, restored building but it was his whole identity). I had no words.