I will let someone more knowledgable answer the UK piece, but why are you considering having him take a test during freshman year?
Studies show that students score higher when they are older, and experts advise taking the test for the first time in spring of junior year. With this schedule, there will be enough time to review the fall PSAT and study to improve the score for spring, while still having several dates to take it again before needing to submit.
If you are looking for an understanding of where he might score, the College Board provides practice tests.
I’m trying to think of a good reason to take an actual test as a freshman.