Well, you said that his father behaves this way. THAT would be why he does. At this age, boyz are drifting further from their Moms and closer to their Dads. What they see the father or male figures in his life doing, is exactly what he will emulate. The number one thing that needs to be done--the father needs to change HIS behavior! The child probably feels this is a way to have his father approve and be proud of him. If the father punishes him for the same things that he is doing himself, this will only confuse the child, and may make things worse. The key is that the parents need to behave in the way they want the child to behave.
No I would never suggest hitting a child..but having consequences for his behavior..YES!!
I would just say to stay consistent, and if it helps, put the consequence for a behavior or chore not done in a column next to the expectations. This way it is in writing for both them and the child to see, and there are no questions. It seems really strict, but soon it becomes habit and things get done. (And there is no arguing about what consequence was for what behavior) And you do not give a harsher punishment because you are upset--because it has already been established and written down what the punishment is for the action. I would say the parents get consequences if their behavior is the same way.
As for meds--this is the school systems answer to any child that will not conform or fit the mainstream line. It seems that this is the first thing many jump to.
If the father did not behave this way, and the child is--then I would wonder what else is going on to make him act out...or think there may be something else that needs to be looked at...But just looking at human behavior, it is common for a 5 and up child to want to emulate his father. This could be a good teaching tool for the Dad to see what he is doing and what influence he has over the child. I say, change the fathers behavior, and the child's will fall in line.