No one, congratulations on your son's accomplishment! Sounds like you have good reason to be excited. (Even though the book is not yet finished.)
I don't know a thing about publishing a book like your son's, so I can't offer much advice.
On the other hand, I can now say that I'm a successful self-publisher: more and more, my piano course is bringing in a significant portion of my income, while providing a real service to those who are using it.
And though its format is far removed from what your son is doing--a subscription website with videos as opposed to a book--I HAVE been getting a taste of what the internet can do for authors in general.
Do you get the feeling that your son is serious about being a writer?
Because for me, that's the key--finding something you love to do and settling in for the long haul. It's taken me a long time (I'm now 66) to finally begin supporting my self through my creative efforts. And boy, does it feel good.
And I'm grateful for the internet because, as someone who's fiercely independent (probably TOO independent), being able to make something and then market it on my own, is a godsend.
Anyway, not much advice (as I said), but I did want to chime in with some random thoughts, a bit of bragging, and my moral support, for what it's worth.
I will say that a taste of success makes it easier for me to enjoy the good fortunes of others, and to share in the excitement that you and your son must be feeling.