From a people management/HR perspective, this incident may not be enough to warrant termination, but it certainly warrants a closer eye on this guy. If you're saying this is not the first time he has done something like this, but nothing was documented in the past, then from an HR perspective this is the first time he has done something like this.
Progressive disciplinary action should always be applied in cases like this. First one is a verbal warning (documented somewhere for future reference), the next time is a written warning, and the next time is a suspension followed by termination. Of course, you can always skip the verbal or written warnings if the first offense was bad enough to warrant it; people shouldn't get a verbal warning for crashing a truck and subsequently failing a urinalysis, for example.
The more documentation you have, the less likely you will end up in court, and the less likely you'll find yourself in a position where you have to pay for his unemployment check. Best bet is to document this and start cutting back his hours until he gets right, or gets gone. Either way you win, you can't have people like this in positions of trust damaging your name and reputation in this business, period.