As the title says, I'm looking for a word or idiom to describe a person whose mood changes regularly in a very short time and you never know what's going on in his head.
He can be very optimistic about the world and suddenly being extremely pessimistic after seeing some bad news in social media.
He can be very positive about dating a nice girl, yet being nervous because he "smells" something's wrong in the way she texts him.
He reckons for very long time about quitting his job and seem to be very determined. Once again, a little "maybe" occurs in his head and he change his mind suddenly.