The boy, identified only by his first name, Jonathan, collapsed in front of stunned shoppers and workers as he helped his father put groceries onto a conveyor belt.
An emergency doctor arrived at the Adese Markt in the Harburg quarter of Hamburg, Germany, a short time later but the boy died.
Local police are now working on a case of involuntary manslaughter.
A homicide division which has undertaken the investigation confirmed the father's suspicion that he was electrocuted by the checkout.
The investigation into the incident last May found that shoddy work carried out on the checkout had left electrical wiring in contact with metal parts of the till, which electrocuted the boy when he accidentally touched it.
Describing it as the worst day of his life, Jonathan's father said he had asked his son to help him put the shopping on the belt at the cashier desk.
That's when his son suddenly collapsed in front of his eyes.
The father said: "Suddenly he hung on the opened cash register.
"His eyes were wide open, he did not utter any sound. I tried to grab my son, but I also got a strong electrical shock."