First, there's nothing wrong with a 10 year old ring bearer. It's a lot better than a terrified 5 year old stumbling down the aisle.
I do agree with others that you're complicating this. Ask his parents if they think he'd like this responsibility. If they say yes, leave Cinderalla out of it. Just explain why it's an important job.
Also, people who fixate too much on their theme sometimes forget practical details. Technically, his parents pay for his outfit, which you pick within the budget they give you. It doesn't have to be this way, but it does have to be discussed.