https://www.sightreadingfactory.com
Free resource for now
Keep your budding theorist going through some exercises on this very user friendly site.
https://musiclab.chromeexperiments.com/Experiments
Experiment with composition and learn as you go
http://songexploder.net/episodes
Get insight behind artists and their music through this podcast.
https://www.npr.org/series/tiny-desk-concerts/
Give your ears a treat that your brain can appreciate.