Well, you did it and you can’t undo it, so now you have to decide what you’re going to do next. Are you going to tell him or keep it a secret? Because both options come with serious consequences and you need to be prepared for either one.
If you tell him, he might leave you and honestly, sleeping with his employee who’s probably around all the time is a special kind of betrayal. You didn’t just cheat, you cheated with someone he sees every day and has to continue employing. That’s messy on so many levels.
If you don’t tell him, you have to live with the guilt and the fear of being found out. And there’s always a chance the driver will say something, either on purpose or by accident. Secrets like this have a way of coming out at the worst possible moment.
You also need to ask yourself why you did it. Was it revenge? Boredom? Self-sabotage? Until you figure out the root cause, you’re probably going to do it again with someone else. Get to know yourself better because this behavior is going to destroy your relationships if you don’t address it.