Hello TSMW,
I'm sorry to hear that your son's iPhone was stolen. If Lost Mode has been activated on his iPhone, the phone would be automatically locked, preventing it from being put in Airplane Mode (the phone could still be powered off, though). For more information about Lost Mode and other tips on what to do if your device is lost or stolen, you may find the following articles helpful:
Find My iPhone: Lost Mode
If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support
Sincerely,
- Brenden