My experience working with extremely complex business logic and a domain expert is that the time of the domain expert tends to be extremely valuable, even more so in a small sized company.
He is of course rattling off endless details and nuances to you because it comes naturally to him for one, and secondly because he is likely an extremely busy person. These types of people don't like to have to repeat themselves.
I know this sounds strange but get a decent digital voice recorder like the kind a journalist might carry around with them. Have a sit down and just let him brain dump. While he is talking take notes but only write down the Main Points.
When you are done, tackle a single point at a time and replay the audio sections to recap all of the details that you missed. If you are not an aural person you can dictate the conversation into text or if your office has a secretary or administrative assistant that you are allowed to utilize then ask her to copy the conversation into a textual document for you.
This is the best way in my opinion and you will find that the domain expert will be much more clear and descriptive knowing he/she is being recorded, just make sure he/she is comfortable with this before you do. Further the weekends play havoc on your short term memory, you will come in on Monday and forget critical pieces of information forcing you to bug the domain expert with nagging questions that he has already been over.
Only then when you have the raw information can you formulate use cases and user stories.