Tell him that errors and wrongs are wrinkles that everyone experiences. And that wrinkles can always be ironed out. Size is usually discussed in terms of the ratio of teacher to pupil. I have, however, been in many large schools that are superb. In those schools there was a concerted effort to break the groups of kids into smaller, more personal groups.
Some kids thrive in large-school environments.
I think the most important consideration is the type of environment, large or small, in which your child will feel most comfortable. One of the most popular Teacher Says columns each year is the summer booklist each June. I am always interested in knowing what kind of books kids like and why. Some kids tell me they love books that have lots of girl characters or ones where the author always uses silly names for the people in the story. My daughter, who is in 8th grade, has not had the world's greatest academic experience in middle school.