Dim backlight

My son has a 4th Gen Ipod nano everything works fine except the brightness. Instead of being bright, then after a short while becoming dimmer, eventually becoming black when no buttons are pressed, it only gets to dim before becoming black. We have gone onto the brightness settings but nothing happens when we turn the wheel. Although the indicator recognises that the wheel is being turned it neither goes brighter or dimmer. Are we missing something obvious or does it need repairing?