I've been to two trials where there was a JR Handler. They compete just like the rest of us, no special consideration, etc. Both qualified and one won a special award at our trial (we had awards for certain things like, "top scoring lab, highest scoring working dog, etc" and she won in one of those categories since her breed was the only one represented at the trial). I can't remember what title she earned that day either - I think maybe a CD, and I do remember the last trial, the kid was going for his beginner novice. He barely qualified and had a really hard time in the ring. I think his nerves got the best of him.
As far as rules, I'm not sure if there is a certain age requirement. I think both the handlers I had seen were probably between 11 - 13. I'm not sure if there was a special consent form or anything either. I honestly haven't looked on the applications for trials, since this really doesn't effect me.
My recommendation tho would be to get him back into classes yourself, but take your son with you. Let him be a part of the class, while you mainly handle so that Jack gets used to being around and working with the other dogs while there is someone a bit stronger at the other end of the leash to help keep him in control. But, let Caleb work with him during the class too. It would be a great experience for everyone.
I think it's a great idea and hope you go forward with it. Also, talk to whatever club you are working with and see what they say as far as a junior handler in competition. I'm sure someone may have an idea.