I dust the amps and the electronics every month or two. I dust the speakers every couple of weeks, and I clean them every month or so. (The speakers have a piano black finish, so they look worse when dusty than the rest of the system.)
However, as far as your system sounding better when clean versus moderately dusty, well, I gotta side with your son on this one!
However, as far as your system sounding better when clean versus moderately dusty, well, I gotta side with your son on this one!