Child psychology. Slap him. please help!

My son wets  bed regularly. Don't have any interest in his studied  . Very lezy and irresponsible. Tries to show off to his friends & every time asks for things which are costlier,i.e. Gear cycle with disc brake instead of a common bicycle .Does not take care of  his belongings. Doesn't listen to us. Always do what he wants . Don't accept his fault, tries to cover it with lies to defend himself.  Jealous of his younger sister of age 4. Tries to get his own way by hook or crook.     Used to be a very cheerful and loving clild up to 4 years old. Was a very smart and obedient one. But gradually developed negative character.       I could not give him much attention after the birth of my daughter. This is one of the reasons besides he lost his grandfather, at the age of 4,who was close to him .I and his father always try to make him understand what if he neglectes his studies rather advise but he gets irritated and most of the time we loose temper and scold  and slap him. Please help