This relates to 2002 to 2006 cars, I am not sure whether it also relates to your son's T reg.
On models equipped with a Peugeot anti-theft alarm system, there are specific procedures that should be followed when disconnecting and reconnecting the battery.
Prior to disconnecting, the alarm system should be disabled by switching on the ignition then immediately depress and hold the alarm switch for 2 seconds; the indicator light on the switch should then flash rapidly for approx. 3 seconds indicating that the alarm has been disabled. The ignition should then be switched off and the battery negative terminal should be disconnected.
Always reconnect the +ve lead first, -ve lead last. Once the battery has been reconnected, operate the deadlocking with the remote transmitter then unlock the vehicle. The alarm will be set as normal; the next time the deadlocking is set.
Before starting the engine for the first time, turn on the ignition and wait for at least 10 seconds to initialise the various electronic control units.
Best of luck, and let us know how you get on.
ChicksFan, December 2011