I think a lot of people with the disorder use cannabis. Drug and alcohol abuse is especially high in people with bipolar disorder. I know it was true in my ex-fiance's case. It is often an attempt at self-medication.
The bottom line though is that the effect of THC on bipolar disorder is not known. There just is not sufficient data. However, the effect of many medications that are prescribed for the disorder have been conclusively shown to help prevent and shorten mood episodes.
I have read about some psychiatrists who prescribe medication to be used at the onset of an episode instead of as a maintenance drug. However, this seems to be rare and it requires that the patient keeps a mood journal and knows when to start taking the medication.
If he really has bipolar disorder, then his behavior is disrupting enough to his life that he was diagnosed. I doubt that any psychiatrist would recommend going off medication (except for very specific circumstances like pregnancy). People with bipolar disorder are much less likely to take their prescribed medication than some with say, high cholesterol of diabetes. It is the sad truth, but these people almost always need to be medicated, even if they think their case is "mild".
The bottom line is that the idea that your partner does not need medication is almost certainly his own opinion, not the opinion of his physician.