I spent too much time adventuring and not enough time killing bandits. By the time I got round to it Kivan had decided to leave me, is there anyway to get him back?

Nope. Any time a character gets fussy and quits your party, they're gone for good.
Also, any time you kick somebody out for any reason, and they say something along the lines of "man that was a waste of time, I hate you guys" they're also gone for good.

In both of those cases, they actually literally cease to exist once they leave your party.

... unless you have a saved game from before the time you offended the character in question. If so, you could go back in time, as it were, and play the scene a different way.

In my game Coran left because I didn't do his quest fast enough and I met him again after an area transition battle and he rejoined. But I was extremely lucky, it's better to assume that Kivan is gone forever.