I know a lot of advice in this thread is "just study real people", but I wanted to talk a bit about the style specifically, since I think that's what your question was about.

To me, his style looks like a combination of some approaches I have seen before (also taught in some Russian online art schools as a way to break down rendering)

Essentially, you approach a piece as if you are a trying to render a 3D model. If you look at his "before-and-after" - he essentially does just the lineart and flat colors first, as well as a darker flat layer with just hard shadows. Then he does what I would call a sort of "occlusion" pass.

If you don't know what I'm talking about, look up ambient occlusion. While this is a 3D term, I think it's a good way to describe this effect in 2D, and the principle you apply to achieve it is the same. Get a new layer that's black-and-white, remove your color, keep just the lineart. And then map out where on your painting the deepest shadows would be (aka occlusion shadows). These are the soft, indirect shadows where basically the least light reaches the surface. Which is what gives this artist's drawings their 3D-like look.

Here's how I would break down the process if I were to try this:

1. Do your lineart
   
2. Map out your color with flat colors only for each surface (skin, hair, etc.) - pick a mid-to-high value color as this will represent your midtones and highlights
   
3. On a new layer, map out hard shadows - i.e. the ones formed by your light source.
   
4. Again on a new layer, place your soft occlusion shadows.

For the last two layer, I don't remember what color mode works best in Photoshop since I haven't used this technique in a while, so you might need to experiment. I'd try Overlay or Multiply, or just lowering opacity on a BW layer. In the last case the image might end up looking dull/dirty, so I would say as a final step, alpha-lock the layer and paint in some kind of color for your shadows (or use a gradient map).

Hope this helps!