Why does Calvin call Charles a moron?

In context, moron refers to someone who lacks intelligence.... doesn't belong. Calvin calls Charles Wallace a morin when he perceives his question to be foolish and inappropriate.

“Tell us what you’re doing here,” Charles Wallace said.
“What is this? The third degree? Aren’t you the one who’s supposed to be the moron?”
Meg flushed with rage, but Charles Wallace answered placidly, “That’s right. If you want me to call my dog off you’d better give.”
“Most peculiar moron I’ve ever met,” Calvin said. “I just came to get away from my family.”