Go buy a big gas can full of regular gas.
Open up the fuel rail somewhere, and position a catch can of some kind near it so you don't spill diesel everywhere.
With the rail open, turn the key to RUN, but don't crank it. The fuel pump will start to pump and try to achieve good fuel pressure in the rail. Since you have the rail open, pressure won't build up, and the pump should keep cycling.
It might have an auto-cutoff of some kind (ie, after 30 seconds). If it does, repeat the process until the tank is drained.
Do NOT let it pump dry for very long. You'll damage the pump.
As soon as the tank is empty, toss in a couple gallons of the regular gas, and run that through.
Add more regular gas, and close off the fuel rail.
Change your spark plugs, and pray that your injectors are okay. When you fill the tank all the way up, add a bottle of injector cleaner.
I'd have the fuel filter changed ASAP as well.
I would consider that to be the absolute bare minumum that you'll need, if you're lucky. You may be looking at replacing a fuel pump and injectors as well.
On a related note - and I say this with complete seriousness, and without any intention of being rude or disrespectful - your son needs some further education before he gets back on the road. If he doesn't know not to put diesel in his car, then I'm guessing that he has little to no hope of changing a spare tire if he needs it, or how to jump-start his car if he ever got stuck, etc.. Consider that operating a vehicle is really a very serious task, and that it's unsafe for people who don't know what they're doing to be on the road.