Be aware that your son's brother who doesn't have the cartridge will be able to play all 32 race courses and all 6 battle courses but will only be able to play as Shy Guy and no one else!.
Also Kart customization is not availible for anyone without a cartridge present as well. for the full experience (multi-player included) each person (up to 8 as stated above) needs their own cartride of the game!. hope this helps! :).