Product characterization course for our “Ideas man”

hey guys!

I'm the lead developer at a startup and we're having issues when it comes to feature requests in general and the Characterization of the product – it's a complete mess. My friend whom I consider as a genius have great ideas for our app but it's impossible to understand what he wants, he gives me drawings of "screens" and I need to figure out what is the user input, output, what the API and App should do when you press a button, how does the database looks like etc… in the end I find myself writing the code again and again, changing the database structure because there's something he didn't mentioned and just waisting precious time.

We're using Monday and I feel like he's not using it as intended, too much boards and complication. (I've worked with Monday before and I never got so frustrated) I'm looking for maybe a Udemy course or something that can teach him how to properly characterize, make a feature request, generally just bring him up to the standard I'm used to from other high-tech companies. just to clarify, I love thinking about the code structure, database and everything but the characterisation is just not enough for me to start with the development right away and I can't even estimate the time required for the developers to get things ready.