Bipolar illness is a biological disease. Most patients have trouble recognizing the highs, and for many the lows sneak up on them. Unfortunately, other than giving him some extra time and allowing him to complete his tasks when he is not depressed, there is a limit to what you can do. The lows come and go and are not induced or worsened by your course requirements. He will not be able to function near as well when he is depressed, and the best thing you could do is allow him the extra time he might need to complete his work.
Depression reduces one's ability to learn and concentrate when it is present.
Effective medical treatment by his doctor will resolve the depression in the vast majority of cases. One risk his doctor faces is that the medications used to treat depression can switch your student into a manic episode, so many psychiatrists are more conservative in treating bipolar depression. Ultimately, the student's job is to learn your class work. I wish all professors were as professional and compassionate as you are.