First of all, if your son is a sophomore, so college concerns can wait a year.
Second, my kids didn’t prep at all (beyond understanding whether to skip a question or guess). The time your daughter spent prepping could have been spent doing something she loved (and that colleges would have found more interesting than a 780 versus a 740).
For your son, there are many very selective/top colleges that don’t require or emphasize standardized testing (some do for merit aid).
https://www.fairtest.org/university/optional
This list is only going to grow.
Another resource is the website Colleges that Change Lives.
It really isn’t necessary to live with so much stress. Not only are there many many excellent schools, but the top schools your family is interested in value authenticity and what an applicant can contribute to the mix on campus. Grades and scores are only a way to get in the pool.
Try to not even discuss grades and scores but help him scout out ways to pursue genuine interests.
I hope your son can enjoy high school without thinking about college at all for awhile.
ps As you know, comparing kids can be difficult for them: you mentioned that your son was more “kind” but also that your daughter has been helping him, which sounds kind to me! That said, if she is mentoring and tutoring him, I would be careful about her transferring attitudes, goals and stress to him.