Regarding the differencing of Arms granted by the College of Arms in London, a younger son, if he wishes, could add a mark of cadency as a younger son, but this is not obligatory, and tends to be observed only where there is a possibility of confusion.

The easiest way around that situation in this day and age would be for the second and any subsequent sons to use the undifferenced Arms, but to have entirely different Crests.

Regards.