I'm going to go off on a related tangent; you're going to have to trust me that this is germane to your OP. The resource below discusses a useful technique employed in innovation design called "emergent signal mapping", which is typically used in business design to detect latent market needs and potential disruptive innovations but, in my case, which is used to identify potential topics of interest for DS. You might enjoy the whole document (only 16 pages), but page 10 is the one to consult. In it, you see an emergent signal map, which is essentially a series of concentric circles of different--and increasingly esoteric--thematically linked topics. Your starting point is the centre, and you radiate outward across 5-6 valence shells of topics as you expand your "signal" set.

http://www.alexandermanu.com/alexandermanu.com/Downloads_files/ICchapter11.pdf

For your son, I did a little brainstorming while in the shower, and a trajectory he might like is:
- Level 0 (starting point): clocks
- Level 1 (next ring): Building a sundial --> discussing the relationship between the sun, moon, and the Earth --> stargazing
- Level 2: solar system and life cycle of stars; tides and the moon; weather systems; hemispheres of the Earth; magnetism
- Level 3: introducing infinity and the number line; building electromagnets; power generation; local geography; local history
- Level 4: theology/mythology

Hope this is at least an interesting thought experiment. smile

What is to give light must endure burning.