What we do in our office, to make sure everybody feels supported, is to have rainbow decorations, and badges and stuff for people to wear.
You could wear a rainbow badge, to show that you support, and so he knows to feel safe. You can also encourage others to wear a rainbow badge, or lanyard or something else, to show that he can feel safe.
I did not downvote this comment, but the approach feels a bit more pushed/forced, which drastically takes away from the sincerity. It's great to show support on your own volition, but if everyone is required/pressured to do so, I would question who really is "safe"
When you're being encouraged in a place that supplies your income and with that your livelihood, an "encouragement" becomes pressure.
Similarly, my brother-in-law once bought a rather expensive gift for my father-in-law and asked if we wanted to pitch in for it. He's truly a great person, we get along great, and if we said no I don't think it would have been a big deal at all... But being the one who married into the family relatively recently, it's a difficult thing to push back on. And that's in the best case scenario with someone you know, love, and trust. We don't know that OP has that same kind of relationship with their coworkers
I really appreciate the sentiment. I don't think I would do anything with stickers or decorations without talking to him first because I don't want it come across as performative.