You raise a very interesting question - but I'm pretty sure that the answer lies in the actual dictionary meaning of the word remission.
The word "Remission" doesn't refer to a cure. For example, I haven't had a mania or depression in 20 yrs. I am "in remission" due to the lithium I take. There is no doubt in my mind that I would begin cycling within months (not right away since my cycles were farther apart).
In answer to your question about your boyfriend (and I question whether he was truly bipolar if he has been off meds for a year or two without a moodswing) I found the following on a bipolar disorder neuroanatomy site:
Even in remission, patients with bipolar disorder remain sensitive to external stressors that can trigger new episodes.
Hope this helps a bit, it's a mystery to me that they pulled him off meds. I'm tempted to think that they told him he was in remission and HE chose to go off his meds on his own, but I could be wrong..