I believe in dirt riding as well. Your boy will learn control in an environment where there are no cars. And crashes are generally, but not always, less severe. And I don't blame you for being nervous about him learning to ride.
There is a great book by David Hough called "Proficient Motorcycling" (The first book).
And here is a MC instructor that has been putting out the best videos on YouTube. He speaks slowly and clearly. Good basic stuff that even an experienced rider should review. He also has a paid subscription type thing as well.