That's a fantastic accomplishment!

This is something I have been thinking about for my 14yo, but know next to nothing about.  What is the best pace to start?  

There are multiple study guides for the tests free and paid. Like any other hobby setting realistic goals (projects) is a good way to track your progress and growth. There are also YouTube channels  (like with anything screen the content ahead of time) you two can watch videos together for basic ideas and study for the "Technician" test. The "Technician" level test will be well within you and your son's reach, maybe make this a goal to pass the test together. Learn Morse Code too, they took this out of the modern "Technician" requirements but it's a great skill and helps you reach out further.

Ham is the most practical thing you don't realize is practical until you have a basic understanding.