[Aire valley]

As above.......the hex fitting is a conversion that fits over the original pin fitting....in your case, you have one of each.
Although pin fit wheels are still available, most wheels are now 12mm hex fitting, so are much more readily available.
Hex fittings provide a more secure fit, less likely to slip, and provide better forward drive.
Best to get the conversion for your sons car and both of you run hex drive wheels.
The B4 is one of my favourites...!