What Causes Extreme Aggressive Behaviour In A Child?

Hi.. I am writing about my 9 year old son. For about 9 months ago,he has started with horrible fits of rage. He gets so angry,he makes fist s and is in such a rage. He trys to hurt me..kicking,punching,trying to trip me etc. He is usually a happy,fun,loving boy. Then it s like a switch is turned on. I tell him after dinner,it will be shower then brushing teeth. So now dinner is over and I say OK time for shower...he SCREAMS NO! And the night is full of anger and anxiety. I might say that his father lives apart from us. Casen ( my son) sees him most weekends and usually spends Saturday night with his dad. This weekend he only saw Casen for maybe a hour. Casen has told me over and over again that he wants us to live as a family. Unfortunately Darrell ( my husband) likes his new found freedom life...friends and females all the time. I stress that no female should be coming over late or middle of the night. Casen is confused and I can understand why. Please help me help my son. I am at a total loss as what to do. Thank you in advance for your time Sincerely, Brenda