I wonder as well about building out some mind maps or other structures either to let DS choose from or to create a kind of mix and match arrangement.
So e.g. maybe your have a big circle which has maths at the top. Then almost a circle of different learning things from there: sciences, psychology, economics, politics, sociology, history, languages, all the way round to film, art, music and philosophy.
Obviously he's little at the moment but I think these could be quite useful resources longer term as well.
So then you've got the circle and you then expand it so e.g. maths there's arithmetic, algebra (or maths machines to start with), stuff that needs rote learning like times tables, shape, graphs etc etc. In science you can start with the easy biology, chemistry, physics, then add things like plants, cells, physiology, ecology, zoology etc etc. I'd probably use fewer science words and more descriptors like "environment", "animals" than I've done here.
So it creates a massive visual map of all the areas one might be covering. You could go down to things like poetry, writing stories, writing for a newspaper, other audience etc.
Ideally it would then be laminated or something so you could write on it.
Then when your son has a phase of interest you can use that to guide the ideas - so he's really interested in space or whatever.
Looking at the big circle then you have the "space" bit in physics which is really obvious but then there's also the history of space travel from the history section, and what humans thought about space, and for animals the different animals that have been up in space and the way politics plays into space (space force) and films about space and what they're trying to do. (What's realistic , what isn't etc.)
Obviously your son is pretty young at the moment but I guess I really mean that structuring the way we try and use our imagination can sometimes make it easier to do .
You could also have a big counterpart chart with big broad topic ideas - space, climate change, the Romans, Tudor life, Antarctica, types of bear, transport, weather etc etc etc.
Edited to add: also to say I suspect you and him creating all this jointly would be a lot more helpful for you both than if you make it and he uses it.