I'd ask about the retention rate at the school. If people stick around, it's likely that they can accommodate kids with a wider range of abilities. There's also more to success in school than numbers on an IQ test, so don't automatically assume that he'll be one of the low performers.

Personally I'd rather my child be at the towards the bottom/middle in a challenging and interesting environment that at the top of a class that is not challenging. Others might have different opinions and want their child to be number 1.