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You are much better of setting time of day rules in the Gateway rather than the online account. Your son has probably got around the parental time controls by modifying the network settings on the Xbox. Parental controls in the Gateway work on the MAC address of the device so your son should not be able to bypass the controls. The current Gateways don't need the settings to be set to GMT. They work on the local time zone.
See section 6 in the documents linked below for how to set up parental controls.