A newsletter format perhaps, meant to eventually make the website merely a side bonus. Depending on how they're done, newsletters can be wildly lucrative, far more so than a site like PainScience.

I scanned his site, I see no newsletter. I think that's an extraordinary mistake, to not have been leveraging all that free Google traffic all those years to build up something like that. You use the free Google traffic to make Google unnecessary to your business, that should always be your focus (of what to utilize the Google traffic for) while the freebie traffic is still flowing.

100% this guy would have been better off building a wholly independent newsletter type service that had zero dependence on Google and could be built out via other means (once it's cash flowing you can legitimately advertise it to build subscribers up, it becomes free-standing and self-building as a real business, Google is almost entirely cut out of the situation then, and anything you get from Google traffic is a bonus). The beauty of a newsletter, again if it's applicable to what you're doing, is it's a free agent, you're not going to be dependent on a singular traffic source, you can take advantage of all of them without much fear of losing your business because one traffic source vanishes.

He still has some search traffic, I'd put all of my effort into building a newsletter on the back of what's left, there might still be time to unlock from Google this way while using Google's remaining search traffic to kick free.

As a bonus he can also still sell/promo his books via a newsletter.