With the changes to SAT looming, 2017 focused on the old SAT first and took it November of Junior year, then retook it in January (final version of old test format, which he wanted because he scored 98th percentile in Writing, which was being phased out because apparently nobody cares about writing anymore). After taking a six-week prep course, he scored 200 points higher, so money well spent.
Then he shifted his attention to ACT and took it in April so that he had both SAT and ACT scores prior to HF in June. When we figured out that getting just a little bit higher ACT might mean more academic money at the D3's where there is no athletic money, he retook the ACT in September of his Senior and scored 2 points higher. And sure enough the Net Price Calculators on some college sites are showing more merit money with the higher score. So again, well worth it to retake it.
I don't think PSAT counts for much other than the chance at National Merit Scholar. But some of the high academic coaches will start looking at kids based on the high PSAT as a Sophomore and then wait to see the results of the SAT/ACT when they are juniors. My recommendation is if your son got a strong PSAT score, take the SAT first, and do it early in Junior year. Leave a few months for a retest, then shift to ACT and get both done before the Summer leading into Senior year. Several coaches at the academic schools told us they value the test scores much more than GPA. One told us "GPA will get our attention, but SAT/ACT determines if we stay interested."