If your DS won't listen, there's not much you can do. Buy him the College Board practice book. Maybe tell him you won't pay for him to apply to any school where he doesn't fall in the middle 50% of students for each section on the SAT. Then back off the SAT (but continue your usual encouragement in other areas, like school and ECs).
There is little correlation between your sons' AP English grade and how well he does on the AP exam. I was a low 90 student in AP English, yet bombed the CR part of the SAT. I did, however ace the ACT, so give that a shot and see if hes interested?