Make sure he installs a NEMA 14-50R outlet and wires per the spec for that outlet. It is NOT the same as a 50A dryer outlet, nor the same as a 50A welder outlet. No offense to your son, but residential electricians too often assume they know what is needed and do the wrong thing for an RV. I have seen far too many reports of RV outlets improperly wired by supposedly qualified electricians, resulting in damage to the motor coach.
Here is another site that has excellent instructions and pictures of an RV 50A outlet installation.
RV Electric