Hi Carrie,
Captain Jack Sparrow's Pirate Tutorial at the Magic Kingdom is always entertaining. He typically chooses 4 - 5 kids a session to join his crew. Kids are randomly chosen & everytime I've seen this it seems the kids are very near the rope they lay out to create a barrier for the stage. I'd try arriving early & being near the front. If he has a pirate bandanna or something else pirate related, wear it. Kids that seem excited & have personality are often chosen. No guarantees but it doesn't hurt to try & stand out.
Jedi Training Academy (JTA) at Disney's Hollywood Studios is way cool! The number of participants per session is limited (approximately 15) & they start registration 1st in the morning & stop once the spots are filled for the day (they usually fill up fast during peak seasons). Sounds like this is on your "must do" list so I'd head towards the JTA stage 1st. Registration will take place somewhere near the stage - just ask any Cast Member (CM) in the vicinity. Kids must be present with an adult to register & CMs will ask them a few questions to make sure they can follow some basic directions.
Both the Pirate Tutorial & JTA take place several times a day. The times will be listed on your Times Guide that you can pick up when you enter each park. Disney doesn't post times for either of these online but there is an unofficial website that posts entertainment schedules (including the Pirate Tutorial & JTA) for the parks about a week in advance -
www.wdwent.com. The site typically updates schedules on Saturday. Its unofficial but I've found the information to be pretty accurate & very helpful for planning. Keep in mind that schedules are always subject to change without notice for many reasons.
I'll keep my fingers crossed for your son. Wishing you many magical memories :-) Allison