Unable to defend and graduate because of advisers fickle mentality
Talk to him (I'm gonna make a template out of that one)

Be professional, stay fact-based, don't say it as a criticism (veiled or not) of him, and you should be in the clear. Say something like:


  I wanted to ask you, given your experience on this matter: I had expected that the final period of my PhD, between finishing the thesis and defending, would be quite short. Given that it's not the case, I was wondering: what is, in your expectations, a typical schedule? Am I doing something that hurts my ability to defend sooner? Could I do something to speed the process?


That should open up a dialogue about your expectations, his expectations, your constraints and his, and possibly help unknot the tie you're in.



However, you're not giving us much information about the exact nature of the core issue:


  he has been postponing my defense for a variety of reasons (all of which are true)


If all the reasons given for postponing the defense are valid (I assume “true” here means “valid”), I don't see why you expect to defend. Do you think he's setting the bar too high? Is it merely a scheduling/timing issue? You say that your thesis is ready, has he gone over it, commented, critiqued, discussed it with you? Is the problem with putting together the committee?