A DeSoto father has now been charged with murder -- in the beating death of his 12-year-old son.  Jonathon Potts died on Tuesday.  Terrence Potts tells police he had spanked his son the night before.  Doctors found Jonathon's back covered with welts and bruises resembling the shape of a looped extension cord.  Terrence Potts was originally arrested on charges of causing injury to a child.  But the charge was upgraded to murder yesterday -- after the autopsy report showed that Jonathon's death was no accident.