Here are some possible explanation for the behaviour. There might be other urgent fires burning for the past week and he is getting nervous and trying to figure out whether he needs to reschedule. He also has to manage up and set expectations when the whole project is going to be done. He might be concerned you are gold plating an otherwise simple task. He might think you are stuck and trying to figure things out on your own. You could help him by letting him have a sense of where you are, and whether there is something more pressing that he needs to talk about. Perhaps he has access to people/equipment/money that can accelerate your work.