there's not a lot you can do, unless you can find something that brings him out of his "down" state. people who are bipolar tend to get very aggressive or mean when they are low it is part of their illness, when they are felling better they usually regret what they have said or done which pushes them back into depression and it starts again, its a vicious circle. bipolar people tend to operate in extremes. they are either extremely happy, everything is going right and the world is perfect. or they are extremely depressed very angry and they want to die. they can stay in either state for any amount of time, minutes, hours, days etc. they tend to get stuck in negative thought patterns and cant pull themselves out of it, it is possible to distract them out of it sometimes but it can be hard work. avoid topics of conversation you know will get a negative reaction and try to divert their attention to something that they they like or enjoy. my mother has this condition and has had it for the last 20 years, one misplaced word or action can put them into days of depression, or a well placed one can have the opposite effect. drugs to control it often dont work very well, if they take drugs for it and it doesnt help they should go to their doctor and try and find one that helps, dont just stick with the first choice. although they usually only help a little.