explain to your child about WHY they belong to wherever they end up based on their ability level?
You said it well!

They belong, based on ability level.
Tamara Fisher, blogging for Education Week on Sep 12 2012 discusses
The Right Fithttp://blogs.edweek.org/teachers/unwrapping_the_gifted/2012/09/the_right_fit.html. Portia gave great advice (as usual) in cautioning against sharing age/grade.
Did your son cry because he felt intimidated? Or possibly because he was disappointed that he did not find other accelerated kids like himself in the class? Did he feel isolated? Left out?
For accelerated kids (whether subject accelerated or whole-grade accelerated), these issues can pop up throughout life... especially at milestones such as the legal age for driving, voting, etc. As kids get older, they may become more adept at deflecting the conversation when it comes to the topic of age, and more confident in their ability to find common interests to focus on.