With sixth grade, a festival really shouldn’t be about a rating or even a particular performance. It should be about the process of preparing for a performance and the feedback the band will receive. As long as the students are all trying hard (like you say he is), who the heck cares if he’s not playing everything perfectly. He’s a sixth grader. There is plenty of time for perfection.
A solution I would probably consider for this situation is rewriting the snare part so the student could be successful at playing it correctly, simplifying and repeating parts that are more accessible. If the student plays with a lot of heart, pulling him off the part could be pretty crushing. Giving him an opportunity to be successful with the skills he has right now will pave the way for more confidence and learning in the future.