If he's very precise about how he wants it to play out maybe it's better to follow his lead for now, I know DS can get quite upset if he has a firm idea of how he wants things.
That said, DS also LOVES it when I put on an accent and really pretend to be someone else, particularly a complaining customer with lots of demands. It really brings out the giggles.
To encourage or extend I introduce a cash register, make menu or price lists, use dress up clothes.