No start - starter wont engage

It sure sounds like a security lock out.

Try doing a relearn

30-Minute Relearn Procedure
Use this procedure after replacing any of the following components:

The passkey III+ (PK3+) keys.
The theft deterrent control module.
The ECM, see important.

With a master passkey III+ (PK3+) key, turn ON the ignition, with the engine OFF.

IMPORTANT:

This procedure is not available on vehicles equipped with EXPORT configured theft deterrent control modules.
When replacing a theft deterrent control module with a GM SPO Replacement Part, insure the procedure to setup a new theft deterrent control module is performed prior to the 30-minute relearn procedure.

Observe the Security telltale, after approximately 10 minutes the telltale will turn off.
Turn OFF the ignition, and wait 5 seconds.
Repeat steps 1-3 two more times for a total of 3 cycles or 30 minutes.
With a master PK3+ key, start the vehicle. The vehicle has now learned the key transponder information and the ECM has now learned the fuel continue password.

IMPORTANT: The vehicle learns the key transponder information and/or passwords on the ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.

If additional keys are required to be learned, refer to Adding Keys (Domestic) Adding Keys (Export including Canada).
With a scan tool, clear any DTCs.

He said the security light is not flashing so no security issue. To me it sounds like the battery has shorted out internally, I would remove it and take it to the auto parts store for testing. Here is how. Check out the images (below). Let us know what you find.