Make it look real? That's a real tough requirement. Hollywood spends millions of dollars and thousands of hours making their effects look real. You're not going to duplicate that on a home video editing system.

You can still have lots of fun though.

You'll have to shoot the video of your son against a flat, even color. Green or blue are often chosen because those two colors don't usually appear in the person you're filming. Choose a color that isn't in his skin, hair, glasses, clothing, props, etc. Make sure it's very evenly lit. Have your son pose in such a way that he can match the frame and action in the Star Wars clip closely. Size and position aren't very critical, but make sure he's facing the right way, looking in the right direction, etc. Pacing and timing are also important. Have him rehearse to the video a few times.

Once you've got him on video, you can place him on an upper track and the Star Wars on a lower track. Add the Chroma Keyer effect to your son's video clip and select the background color. You may need to tweak the settings a bit to get most of the background gone without erasing him. Use Pan/Crop to move him around in the scene and place him where you want him.

Note that using this effect you are pretty much limited to placing him in front of the scene. If you want him to move behind the characters then you have tons more work using compositing and rotoscoping. This is probably beyond what you would want to attempt in MovieStudio.