The only thing I could think of here, assuming all else is equal (antenna pointing the same way, same power levels, same everything else on BOTH ENDS of the QSO) is the audio frequency in use.
Perhaps your son was low in the audio range, down around +500 or so, and you were much higher +1000, +1500, etc. If the rig's audio is not quite linear, or if the soundcard is a cheap one with some noise, then it COULD generate some extra "noise" in the transmitted signal if everything isn't smooth across the spectrum...or if the audio processor on the rig is left on, you could be having some problems.
Just a guess as unless you can say that EVERYTHING is exactly equal, it is hard to know for sure.
Or, it could just be that the "gentleman" has something against your son.
73
Dave
K3DCW