Since Purdue superscores the highest sections of the SAT then if your son scores at least 700 on the English part he should stop. Because his total score would be 1450, which equates with a 33 on the ACT on the concordance table for 2018-19. And with that SAT (and equivalent type ACT score) he would be competitive for large merit aid under the holistic formula that Purdue uses. If he doesn’t score at least 700, it may be worth it to try the ACT—the Princeton Reciew gives a free diagnostic test and he could see how he scores on a practice test. IF he does great then he can try for the September ACT. If he scores meh and /or he doesn’t feel better taking it than the SAT, then he doesn’t need to take it. He could try one more time for the SAT without a tutor. Use his notes from his prior tutor sessions. All the lessons will have sunk in and you may be pleasantly surprised that he does better.