Contacting him isn’t the problem you need to attempt to tackle right now.
The problem you have is that we don’t have a spokesman - the person who you imagine will speak to Joe. Also, you haven’t worked out what you’re going to tell Joe you’d like to talk about.
The number of people who’d love to get an appearance on a production with an audience that size will be large. He is unlikely to be struggling to find guests, even with the prolific number of podcasts he’s putting out.
It’s a pipe dream to expect him to say yes to a complete unknown saying “hi, we’ve got PFS, could we talk about it on your podcast?”
Look at the guests he gets on his podcasts. Famous actors and athletes, arguably the most famous businessman in the world. His audience downloads the podcasts for that. You’ll struggle to make a medical story featuring an unknown a compelling download.
If you could find a celebrity with PFS who would like to talk about it, there would be an angle there, a story. What’s your angle without it? You need to work out what the story is.
As I said, I think it’s extremely unlikely that Joe would think that his audience would be sufficiently engaged by your current pitch (is there one?). You need to seriously think about what is in it for him, the podcast is a business concern. The advertising on the podcast pays for his house and his children’s food.
I only say this as if by some small miracle, Joe does consider this a cause he’d support in some way, wasting his time by not having any idea of what you’d like to talk about would mean he’d be far less receptive when you do have a good idea.