Anti theft system

You only have one chance here and that is the relearn procedure. If this doesn't work, it means that there is a hard fault and your will need a Tech2 scan tool and I'm will to bet he didn't leave you one of those. Your going to be stuck with either the dealer or electrical specaility shop.

30 Minute Re-Learn Procedure
1. Turn ON the ignition, with the engine OFF.
2. Attempt to start the engine, then release the key to ON (vehicle will not start).
3. Observe the SECURITY telltale, after approximately 10 minutes the telltale will turn OFF.
4. Turn OFF the ignition, and wait 5 seconds.
5. Repeat steps 1 through 4 two more times for a total of 3 cycles/30 minutes ( the vehicle is now ready to relearn the Passlockà † Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK).
Important
The vehicle learns the Passlockà † Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
6. Start the engine (the vehicle has now learned the Passlockà † Sensor Data Code and/or password).
7. With a scan tool, clear any DTCs if desired (history DTCs will self clear after 100 ignition cycles).

This is not something the average mechanic will have. It is a factory GM scan tool and with it costing $10K plus and only being good for GM, it's not that common in the aftermarket.

GM PASS LOCK RELEARN- Ignition to "run" position, try to start and let key return to "run" position.
Approx ten minutes security light will go off
Turn key "off" and wait 5 seconds
Repeat 2 more times for a total of 3 relearns.
Turn key off, then start it.

GM anti-theft systems are real effective at keeping owners out of their cars. They don't do much to deter thieves. One common problem is broken wires to the sensor in the ignition switch.

You may have to do security relearn 3 times.