As I understand it, non BRCA members who have not raced before are allowed to race at an event without joining the BRCA for a maximum of 3 club meetings. So your son should have been fine.. It's a shame both you as a BRCA member having received a BRCA handbook and race control were not up on the BRCA rules.
See page 20 of the handbook.
http://www.brca.org/sites/www.brca.o...2014%20(1).pdf