Just a thought but maybe its not that you don't have the right settings, he may just not have anything to cough up. I know I didn't have a productive cough until my late teens.
But to answer your question, when I had my 1st vest, I did 3 sets, 10 minutes each, on 12, 14, 16. When I got the smaller vest, I bumped up the frequency to 13, 15, 17, so it would feel like my old vest. The pressure is set on 6.
Hope this helps!