If this is the first time that he has made a mistake of this magnitude, and typically does good work, the worst that happens should be a written warning to document that it cannot happen again. When people deliver good work day after day and something happens that is clearly a mistake and didn't happen through their negligence, I usually just say something like "you know this cost us a lot of money and time, I am sure you feel badly and I sure that you will make sure that is doesn't happen again. If it does, I will be forced to take more serious action but I don't expect that to happen".
I can't figure out managers who always have to have someone to blame. You don't get the best out of people if they aren't allowed to fail occasionally. My mantra is 'if you didn't kill anyone, we can fix everything else'. :-)