IMO would submit SAT over ACT due to the Math score which is important as a prospective engineering major (Science is more of a reading section than actual science like the SAT Subject Tests.) However, a strong SAT Subject Test or AP test score can counteract this.
I would not submit both scores, as they’re basically the same and it costs more to submit both.
If he decides to submit the SAT only, I would consider deleting his ACT score so that he doesn’t have to submit them to schools like Georgetown that requires ALL test dates b/c once it’s deleted it’s as if it never existed.
Scroll down to “Sending Scores” on this page: https://www.act.org/content/act/en/products-and-services/the-act/help.html#scores
Keep in mind that it takes a few weeks for this to occur, as ACT has to send you back the form and you have to send it to them.
Hope that helps! Good luck with admissions!