What Is The Cause And Treatment For Bipolar Disorder?

Hi, My boyfriend has been diagnosed with Bipolar disorder about 7 years ago. He has been on and off different types of drugs. It was such a nightmare living with his manic episodes that i moved interstate. Ever since i have left, he has been really stable. He is on an anti depressant and he hasnt had an episode for 1 1/2 years. What are the chances of him relapsing? I have known him for 15 years and he has always been in major highs and lows.