The obvious way to try and cheer him up is to ask what he'd like to do that would make him happy. Maybe you could play tennis for some exercise or go to the beach or something else to get him out of the house and thinking about something different.
But if he has a serious problem with depression, it is probably wise to try encouraging him to see his doctor. There might be a medical problem causing his mood swings and it won't be fixed without some kind of intervention. Tell him how nice it would be if he could feel great every day just by taking a pill. That might get him to head to the doctor for a diagnosis.