I think you'd be better finding someone that works almost exclusively with investors, unless his 25% is actually a huge amount of business. I think it's usually a very different person that's good with the investment side than the person good at retail. I would ask the following:
How long have you been working with investors?
Have any of your past clients have executed a BRRRR strategy? Do you know what that stands for?
Do you have recommendations for property managers, inspectors, lenders, and contractors?
Do you invest yourself?
Do you have any formal negotiation training?
What parts of town have the most opportunities for my strategy?
What is a reasonable expected response time for your questions? The number one complaint clients have about most agents is their responsiveness. It's good to know upfront what the agent's standards are.
What is your typical deal size?
Why do you like working with investors?
I'm sure I'm missing some, but it's a start.