Novgorod was the only major East Slavic settlement to avoid destruction or subjugation by the Golden Horde, so I think it is akin to a boy from a well-to-do family in medieval Avignon or Strasbourg learning to read and write. Meaning, not just any city or any family in the mid/late 13th century had the need or means for such schooling, but as pointed out in this thread it was more likely in Novgorod.

Well, probably not most children. I don’t really know anything about that particular region at that particular time, but based on history generally, literacy was - until recently - often reserved for higher social classes.

It's not clear how old he was.