A good coach will recognize what's working and leave it alone; a good coach might also notice what isn't working amongst the hundreds of factors in defensive and offensive skills and offer your son some assistance. Talk to your son about always listening to Coach, no matter who Coach is. Coach deserves respect by his position (and I know there are coaches who have proven themselves not worthy of respect, but that's another topic.)
I believe going into hs with the attitude that the coach will be detrimental (perhaps because of gossip you've heard) will not do your son any good at all, as a ballplayer or as a person. Life lesson time here.
A bad coach may well not know enough to recognize when your player is assimilating his ideas, as long as your son is successful. He should listen intently (and, as Brod says
) smile and nod, then quietly continue to do it the right way.
Should your son struggle or slump, it's time for adjustments. Whether they come from you, your outside coaches, or the hs coach (blind squirrels and all that) is a balance your son will have to strike.
When my son faced this problem, I believe he handled it well. If Coach said something (shall we say) ill-advised while instructing a group, my son would say something to the effect that "I can see why you're saying that. But if we do it that way, what if X happens. If we so it this way, then (here are the benefits) What do you think?" Coach may reconsider his instruction, he may still shut your son down; but there isn't the confrontation of "no" or being arguementative.