Hello, Aya,
I double-checked the park hours at Disney's Hollywood Studios on July 26, 2015, and I also didn't see any specific times listed for the
Jedi Training Academy. While the event is typically offered everyday at various times, I would opt to try and attend on July 25, 2015, since the park hours on that date do include the times of the Jedi Training Academy.
On July 25, 2015, I would arrive at the park before it opens and then head directly to the location indicated on the
Times Guides (located with the park maps) to register when the park opens. When I was just there this last month the registration was at either the Epic Stunt Theater or the ABC Sound Studio. If you are unsure of the location upon your arrival, a cast member can help you you, too.
May the Force be with you and your son!