I would not recommend a motocross helmet. Aside from the added weight which may add to the "whip" effect in a tumbling fall, there is also the visor and extended face protector that may catch in a roller and again, add to a neck injury.
USSA & FIS will not allow helmets with such projections on a race course. The only exception is in slalom with the mouth guard. One company has a low profile guard and that is the way I'd go.
Explaining why he chooses to wear a motocross helmet is a thread all of its own, and should probably be posted in Humour.