You have bigger issues and risks than just gathering requirements. A new system in any organization means a transformation from the way they currently do business to the new way. The change all by itself is a source of major angst and change resistance of any organization, even of the project is moving along well. One of the major critical success factors in organization change management (OCM) is an engaged sponsor, as indicated by level of participation, commitment of resources (human, funds, space, etc.), and other favorable behaviors.
Gathering requirements is the beginning of it all and will likely dictate the rest of the project. If your dad is exhibiting this behavior now, it will likely only get worse, and then get exacerbated when your best assumptions of what is best for the business are wrong.
Your solution is not to find ways to arrive at getting your requirements. You need to fix the underlying issue first because this will wreak havoc on the rest of your project.
You need to talk about sponsorship with your dad, what it means, the commitment, follow-through, etc. He needs to understand what it means to charter a project, to develop the scope statement, to put in place a PM and a dedicated team, to fund it, to stand behind it, etc.
On your side, you need to learn the fundamentals of OCM.