That being said, there is a plenty of free content on the site, so it is not necessary to pay anything.
It's good that you offer free content, I might try it but I wanna know, is there a significant difference between free and premium tracks? And is there really any evidence that they help with the focus? Are they good for a person with ADHD or ADD? How would it compare to Ritalin, Vyvanse or Adderall? To be honest, I am very skeptical in this case but since you have a free plan, I might give it a try.
You should probably get a doctor to recommend this. People are more likely to try/buy if it's backed by something factual.
How should he get a doctor's recommendation? The only thing that comes to my mind is to hire a group of doctors to make a research and publish results on ncbi.nlm.nih.gov but that would cost a fortune.