For a totally unbiased opinion, assuming you have access to clubs that race every class, GT12 is the most accessible and cheap class around. Buy a Supatox or Mardave, add the Core-RC motor/ESC/battery set and you have something that is capable of being very competitive at club level while being easy to set up and race, with plenty of advice from the GT12 Facebook group. Running costs are minimal and the very few parts you can break are pretty cheap and readily available direct from the manufacturers if you can't find them locally.
Although asking what is the best choice of class is pretty pointless if there is no club near you that runs it. Someone suggesting off road is pointless if all you have are touring car clubs, and suggesting on road is pointless if all you have are off road clubs. Look through the club lists at
Central Booking, the
BRCA and here on
Oople to see what other clubs are near you and take him round the clubs to see what takes his fancy.