It is not unusual for addicts to begin making excuses to leave the first week or so of rehab. They have usually detoxed and are now craving their drug. They will play on our sympathy or fear to get an okay to leave and then they will pick up where they left off, once out.
The good news is that most make it past this difficult time. I really hope that is the case with your fiance. The longer he stays, the better his odds of learning the skills needed to embrace recovery.
Keeping you both in my prayers.