Your son will be competing against all other Junior shooters for the category.
He is also competing against EVERYBODY there for Division and Class awards. So there don't have to be any other Juniors there for him to compete.
There are videos all over the www.
IDPA is a time plus competition.
When the buzzer sounds, you draw your gun and shoot the targets as prescribed. Your score is your time from the Go until your last shot, plus half a second for every point you lose on the target by failing to hit the center circle or headbox, plus penalties for not following the course requirements.
Nearly every stage is different. Every match is different. You have to know IDPA rules and procedures and you have to pay attention to the individual stage requirements.
Don't be intimidated by it, your son can explain it to you just like he does the computer. I shot the National Championships on a squad with a club member and his 13 year old daughter. Her opening statement: "I won't be in last place." And she wasn't. When the Safety Officer asked if anybody was willing to go first, she said: "I will."