He can leave right away but this new rule could affect Kentucky scholarships in the future with him leaving in the middle of the school year. This I got from the ESPN article.
Under the new APR, schools are subject to scholarship penalties if student-athletes leave early and not in good academic standing. Withdrawing from school before the end of a semester can seriously impede a team's GPA and academic standing. To that end, Texas and Ohio State are taking steps to ensure freshmen Kevin Durant and Greg Oden will finish the spring semester.