Reddit mentions of Nasco 9725407 Foam Geometric Solids Set, Six-Piece, White

First: get yourself a nice big pad of 18x24 newsprint, some charcoal (a mix of soft vine charcoal and compressed will give you nice range of values and edges to draw with) a kneaded eraser to pull out highlights and correct mistakes, and a chamois cloth (get it really nice and dirty with charcoal) for laying down smooth tones. It's messy, it's weird, but it gives you much more range than a pencil and costs almost nothing. Working larger and sitting further away will help you to form the habit of drawing from the shoulder for smoother line quality. You want this, especially while you're in the early learning stages. Your first step is to teach your body and eyes what to draw, not how to draw X Y or Z or even how to tattoo. That comes much later, and I'm glad to see you already knew that.

Before you jump into the figure focus on basic geometric forms - basic white cubes, cylinders, cones, spheres, etc. Draw them from life, not photographs or imagination. Something like this would be great. I know, they suck - every student I've ever had has let me know, but now they're off making Big Hero 6, League of Legends, and many other awesome projects and would tell you the same. These basic forms help you by reducing variables so you can focus on how light wraps around the form. The difference between a highlight, form light, form shadow, a core shadow, reflected light, and a cast shadow is difficult to pick out when you're dealing with lots of subtle plane changes. This is where you learn the basics of value control to distinguish between different levels of light. Lay down these values with masses, not individual lines. It's kinda half-way between drawing and painting with a broad brush by using the side of the charcoal rather than just the tip.

Certain types of light/shadow produce different edge qualities, pay attention to how hard or soft transitions are from light to dark and think about the physics behind it to understand why - pinning up some light fabric on the wall with a single light off to the side is a fantastic way to see this - the cloth catches the light, creating form and cast shadows as the light wraps around the folds and reflects a little light back onto nearby surfaces.

The goal here is to understand how to represent the light accurately to describe the form (that is all you're doing as an artist- recreating light to trick the eye, it doesn't matter what the subject is) When you're feeling confident with this (and probably filled up 1-3 of those newsprint pads) move on to still life set ups and introduce some more complex forms and add a dash of color (1-2 at a time, keep it simple) Feel free to start playing with the figure, but you'll be hard pressed to find anything more complex and subtle to attempt. Without a teacher present, Andrew Loomis books will be an invaluable resource once you get to this stage. They're free online as PDFs for the most part.

Can someone explain why this sub seems to like these things?

I am of the opinion one should learn something the analogue way until experimenting with a tablet. For one, I find digital art to be lacking texture and sometimes emotion. People wonder "did this person just put an image into photoshop and trace details?"

I would highly recommend spending $20 mastering these basic shapes under a single light source. One could even use an egg, a marshmallow, a shoebox, etc instead.

A 9 year old could draw an accurate sphere from observation no problem even if they can't yet understand the theory of light and form. By this observational exercise, the theory diagram will be self-evident to your son and intuitive over time. Later, the inverse of these forms (a bowl, the inside of a shoebox) are equally important.

Given this knowledge, your son will be well equipped later on, and perhaps the tablet could be a gift for going to middle/high school.

tldr: IMHO a tablet offers little chance to improve one's art compared to a marshmallow.