Do projects. Ask your son what he thinks would be a fun thing to build. Assess its feasibility. Make it happen!

As you've guessed, this is an accessible field, and there are a lot of entry points and sub-disciplines. Some thoughts:

  - Avoid batteries and anything higher than ~24 volts; keeps things safe given your son's age
  - Teach proper safety when handling chemicals like flux, and hot-temperature items like soldering equipment. Set up a well-ventilated environment, and make sure your son knows that inhaling fumes is dangerous long-term.
  - Don't use leaded solder (Fight me!)
  - Install KiCad. Let your son mess around with it, using your (Or even better: Documentation!) as an example. Order his creations from Shenzhen! They don't have to work. It will be very cool for him to make a fun-shaped PCB with a crazy silkscreen or copper drawing, some lights or w/e, and have it arrive 10 days later!
  - Get a dev board with an MCU of your choice, and teach your son to program int. STM32, ESP32 etc.