Chastity Planning and Warning
Tell him several hours in advance about being having to wear the Chastity Device, giving him a specific time when it must be on. Giving advanced notice intensifies the fear of anything, causing him to think hard about what’s to come (or not “cum” in this case).
You can tell him directly and clearly, or you can use some sort of silent symbol that only he will understand like painting your nails a certain color or wearing special earrings that symbolize chastity locks an keys, etc. You can train him with these indirect symbols and then use them when you are with him in public as you watch him get extremely frustrated and fearful when he sees them, while anyone else around him will never know.
I don’t know about fearful… seems like just a way to get him horny! I think putting on a necklace with the key is the obvious choice!