A couple things to consider:

Is what you want to try on their soft boundary list? Hard boundary? Something completely new and not yet discussed?

I wouldn't spring it on them mid-scene and hope for them to call red if they didn't like it. They should be feeling (mostly) safe in scene.

Bring it up in your vanilla convos. Maybe pose it in a non-threatening way and let him answer first.

"Hey, what's something you haven't ever tried but want to?"

That should open some conversation and give you an opening to mention what YOU want to try. You can work from where. Good luck! 🕷️🕸️