Depends upon your son but perhaps youtube + forum engagement with others centred about math channels that have lower+upper school and university material.
> It follows that we must not introduce any topic for which we cannot first convince the students that they should want to pursue it.
There are several good math related youtube channels and they have the advantage of allowing viewers to find questions | topics that they find of interest and then pursue .. which can lead to following for the latest releases and discussing with others that comment.
Matt is very engaging and there's a fair bit of back and forth from others on his channel comments which might take your son down a path of greater engagement and exploration.
> It follows that we must not introduce any topic for which we cannot first convince the students that they should want to pursue it.
There are several good math related youtube channels and they have the advantage of allowing viewers to find questions | topics that they find of interest and then pursue .. which can lead to following for the latest releases and discussing with others that comment.
Matt is very engaging and there's a fair bit of back and forth from others on his channel comments which might take your son down a path of greater engagement and exploration.
https://www.youtube.com/user/standupmaths