I still don't quite understand how the actual competition works. The 12-18yo would be his "group", so would he actually have competition at the local club?
Depends on the local club. I have seen younger shooters at local matches and they do just fine. If there are no other shooter in is age/group he will just compete as the adults do.
He will score as everyone else does, raw time plus any penalties he receives on the stage. Being he is not classified he when scored will be noted as unclassified.
There should be 6 stages set up for the match, each with a different scenario and each will have the possibility of how the targets are engaged: tactical priority, tactical sequence and the possible limit of rounds you can shoot on a given stage. Vickers count or Limited Vickers.
The on-line rule book can explain further and if not just ask away.