…that I was unprofessional and I would regret passing up this opportunity.
I'd tell him that the word "Professional" only applies if he was paying me.
I've always heard from every artist I know that they quickly learn to never do something for free.
The only time I'd trust that exposure alone may be enough is if I know who they are off the top of my head with no trouble. …and I mean them personally. Not someone they claim to know.