broken goods sell to customers

My son bought a K300 Quadcoter at Homemark in January. After 2 days one of the rotors stop working. We took it back to the store within 7 and they send it in for repairs. After more than 4 week and numerous phone calls we received it back. 4 days later the same rotor stopped to work. Again we took it back and they replaced it. My son did not want it back and they did not want to refund it it. He decided to add some money and bought the Roto Quadcopter drone X129L but a few days later it did not work. We took it back AGAIN! they wanted to send it in for repairs but we insisted on a replacement. The replacement one also stop working after a day. It would only be ear to refund him for all the misery caused not to be able to play.