What to do if boss always postpones major decisions about requirements and overall design?

Question

When starting a new project, my boss always avoids to make fixed decisions. He is usually saying: ok, just start to write something and be as generic as possible. When you're finished we look how we continue. His argument is basically that you never know and "agile development".

To keep the question as generic as possible: what do you do if your boss doesn't like to make decisions?

Just stick to it and write code that might undergo heavy refactoring and partial rewriting a few weeks later? Or keep on discussing until the boss does at least a few decisions? This is more or less my current strategy. Because it's like a law of physics, at some point something needs to be delivered. Either because the boss' boss wants to see results or because stuff is becoming ridiculous at some point.

I also observe that my boss is criticizing nearly everything. Even suggestions that are based on his own...