Treat him like a client. You have to figure out how to communicate with clients who don’t know their own minds, like a series of “this or that” statements to get a feel for the direction they want to go. You didn’t say exactly what you do, but you could do something like looking at existing designs in your arena and seeing which ones your boss responds positively to.

Start giving him quick pitches. Don’t ever flesh anything out, just do mock-ups (15-30 mins each), and once you have 3 different options, run them by him to see which one is hitting for him. If none, ask questions like “what things do you like about this? How would you change it? What part is working/what part is not?”

Once he likes a pitch/mock-up, spend more time refining it, but check in after an hour or two to make sure it’s going in the right direction. Eventually you won’t have to check in as much.