Went through this with more than one big bully as a kid, and have watched such situations evolve to the point of ridiculousness in the current school scene.
Was just average in size but very big on "don't touch me", and was raised the same as huntsonora. Never picked a single fight and kicked butt rather heavily the few times the bully hit or pushed me first. The teachers/principal did talk harshly to me, but they were smarter than today's "educators" and knew these things are best left to the kids.
Since you ask, my thoughts are: do not get yourself involved with the dad or kid, verbally or otherwise; advise your son not to instigate an attack merely in response to lies or insults - let those go because anyone with sense will know the bully for what he is; if he is going to respond forcefully, he should do that away from the school and, preferably, away from any crowd or gathering; if he decides to respond to some physical attack, he should strike suddenly, hard and relentlessly until the other kid is down, crying and whipped. Then walk away.
