Have a code review before every commit (even if it's a 1 minute "I've changed this variable name"), and as part of the code review, review any unit tests.
Don't sign off on the commit until the tests are in place.
(Also - If his work wasn't tested - why was it in a production build in the first place? If it's not tested, don't let it in, then you won't have to work weekends)
