My 7 year old son ot do do this in June! It was great.
Get there in advance of the show. Once the music starts and the CMs come out, it gets crowded. As you are facing the stage, sit to the right, that seemed to be where the show was aimed.
We got there early and I laid a towel on the ground for my DS, DD and I to sit on. We sat right behind the line that is on the ground, (you have to stay behind the line.)
I told DS that he might or might not get picked BUT if he wanted to get picked here is what I told him that he needed to do.
"As soon as the CM comes out everyone stood up. When they ask for volunteers, you need to raise your hand right away, don't look like you are unsure or like you are too cool to be in the show. Smile and look excited, but don't act crazy, like you don't know how to follow directions."
The Jedi instructor trains them and they all use a light saber and put on a robe. I think that they picked about 12 kids. Then, the music changes and there is smoke. Darth Vader comes out and the Jedi instructor herds the kids to the right side of the stage.
Each kid battles Darth Vader and he says something to each one.
My 7 year old said that when he had to fight Darth Vader, that even his legs were scared!

There was a photopass photographer there and she got 2 awesome pictures! We even bought one that day to show dad who was working at a conference.
It was intimidating for the kids I think. I noticed that they had one of the older kids fight Darth Vader first. Probably so that the others could watch and see what happened.
Your son is the perfect age. The youngest kid that was picked the day we were there was probably 5, maybe 6 and the oldest was maybe 10. If your son seems at all unsure, let him watch a show and then come back for a later one.
This turned out to be his most memorable moment at Disney.