That in general is the case. If the ACT score is higher than SAT, then you just send in the ACT score.
Here the thing gets a bit more complicated. If your son is applying private schools, then he may need at least 2 SAT II scores. Even some public schools require SAT II. You can probably submit SAT II scores and suppress the SAT I score that you don't want to send with 'score choice'.
Make sure that you check the university's policy towards SAT 'score choice'. Some of them say 'score choice' is okay, others demand all scores.
