The NSS is located in the shifter console, to the left of the shifter.

Here is what it looks like.

http://www.partsgeek.com/mmparts/neutral_safety_switch/volvo/940.html

You can test by putting a test light on the solenoid terminal on the starter and try to crank- no light means no power to the solenoid.

Replacement is not a difficult chore - remove the console, couple of small hex screws and a plug. The mounting plate has elongated holes to allow for some small positioning adjustments. Before you buy, try adjusting the switch position to see if that cures the problem.

You can test for a defective ignition switch by unplugging the NSS, jumping the blue and pink wire in the plug, and trying to start the car. If the car cranks, the ignition switch is okay and the NSS is the culprit. You can also put a test light on the pink wire and turn to the start position - if you get a light, the ignition switch is okay.

If you need the car while waiting for the part, you can leave the pink/blue jumper in place - just tell your son to make absolutely sure the car is in park before starting. Also, if jumped, you'll lose the reverse lights.

Another indicator of a faulty NSS is reverse light failure.

NSS can be found at any of the usual suspect online suppliers - $40-$60.