If you think he's going to let it expire, gently let him know that what you're willing to pay today is cash in pocket. If he isn't going to use it anything, whatever you pay him is bonus money to him.
Does it currently have an active website? How old is the domain name?
These are important questions to know the answer to. If he has put a lot of work into it
he may not want to lose it to an anonymous registrar. If you can make him comfortable with your plans for the domain he may be more comfortable with selling it to you. Most of us have put our blood, sweat and tears into our sites. It's like letting your child go. Make him feel good about you as the next phase for this domain.
Please let us all know how this turns out. It's very interesting