mine! Came here today because I have a similar issue on a 2000 5.4L
On your son's truck, when the heater starts blowing cold, try revving the engine above 2500rpms (or if riding, turn off Over Drive, or downshift to get the rpms up). Mine at this point will start blowing scaulding hot air and the engine temp will drop back to normal. Always happens in first 3-5 miles on a COLD start. The rest of my one hour commute goes perfectly smooth once I get the t-stat to open. I've replaced upper and lower hoses, belt, waterpump, and THREE
thermostats. Issue is somehow related to the pressure in the system not being adequate to open the t-stat. God help us, I know of nothing else in the system I have not replaced yet.
Since Christmas, I've just been driving the begining of the trip with the OD off for 10 miles and I can't even tell the truck has an issue!