Save as IDML. That can be opened by versions CS4 and later, but there mayh be loss of content due to unsupported features and line endings may change in the text..
Better would be to just work in CC 2014 if that's what you need. It's included in your subscription: How to Find and Install Previous Version of Adobe Apps in CC 2015 | Adobe Customer Care Team
DON'T USE CAPS!°
You have to export as IDML and the other one has to have the latest update from his version (which one), so he can open it.
Thanks guys and sorry for the CAPS!!
CC2014 should prompt you to use the online tools to convert the file.