My guess is that it isn't technically probation, but rather that when he was sentenced on his DUI, he had jail time or fines "suspended" for a period of time(likely 1 year) under certain conditions, which would more than likely have included no further law violations.
So, the court now would have the option to revoke that suspended sentence and make him sit the time. The suspended sentence may not even be jail time, but fines or fees assessed by the court, which in the case of a 1st offense DUI, is more likely.