Greetings Christina,
I see your young padawan has fallen to the Dark Side of The Force. Lord Vader will be most pleased. Your youngling is most certainly welcome to wear a costume during your visit to Disney's Hollywood Studios Theme Park. I would plan to bring the costume along, with a change of clothes, so you're prepared and don't need to spend time looking for a costume when you arrive.
I highly suggest enrolling your son in
Star Wars: Jedi Training Academy on the day of your visit. He can learn the ways of The Force from a Jedi Master and, perhaps, come back to the Light Side. And don't forget to check out
Star Tours- The Adventure Continues, too. My padawans always insist on taking several trips on the Starspeeder 1000. I think they're secretly hoping we land on Endor for good.
Please feel free to follow up with me if you have any additional questions.
May The Force be with you,
Ashley