I have taught my sons and some cousins to sit once for 5 minutes. And I have sat for 5 minutes here and there with my boys. But this felt more serious and real to sit for a full 20 minutes with my son.
I read in One Breath at a Time: Buddhism and the Twelve Steps that one of the ways to teach children to meditate is to tell them to pretend to meditate. You can kind of sheep dog them over time towards what you want.
Next step is to teach him Metta, and get my younger son involved.