"That should be an easy fix to replace the key switch. If that doesn't work, if you're not a mekanyk (I can't even spell "Mechanic" so I guess I ain't one!) take it back to your guy and have him fix it.
Technically, since it worked when run in the driveway, I agree it is the switch. But, verify wire connections to the switch first. After that, I wouldn't hesitate taking it back to the guy who worked on, either.