oh sorry to hear that ..how old is he? how long is he racing?
i agree w/brett good advice to ask him what he likes about racing and what he dosnt… if he can make a list of good/ bad things it helps to iron out what is making him change his mind. if its the comp thing or if he is constantly in the back of the pack maybe focus onthe team thing instead of individ racing.. it can help if they are feelling like they suck( happens to everyone) then if they join a fun team ( not interested in team sheets/ points etc just the social end of it) it can change things..good luck