Sorry to hear that the "quick fix" didn't work out. In the past I have learned that Barrs Leak is the only nearly safe product to use for something like this. I have used as many as 2 or 3 cans/jars of it in one application and with our trucks the only way is through the main or upper radiator hose to the radiator and not the reservoir. It's also a PIA to do but not as bad as changing the heater core. Then to insure against clogging the system you need to flush the excess out and refill with fresh coolant. I would never recommend this method except in the most dire circumstances. I hope your son and family made it back home with no further problems.
I have a '98 Expedition that I'm selling cheap that needs a 4.6 engine only, the rest of it is in great shape. Why don't you buy that for him? An engine will run under $250 at Pick-N-Pull. A week end of labor and he he could have a great family truck. You have my number.