First step is under the table. Make him install an app or bring a cheap phone yourself preloaded with a good wallet e.g. Cake that you leave him for the season.

Step two: Come back with friends regularly to make him aware itvis worth to put some time into this.

Third step: Bring some mini server installed with BTCPayServer.

You encounter merchants not in one go. Every metchant is at least a one year project.