My daughter has recently applied to universities here through UCAS, and so far it seems like a pretty good way to go (less stressful on the whole than the U.S. application process.
I've told this story before, but she came here at age 17, which is a dreadful time to slot into the U.K. education system. She was able to graduate a year early in the U.S., but we were told that U.K. admissions would only be interested in A.P. test scores of American applicants: GPAs and SAT scores mean absolutely nothing. Unfortunately, her American high school was an alternative school that did not offer A.P. courses. She took some A.P exams anyway, without having taken the classes, and did okay but not up to Oxbridge standards. She enrolled in a one-year A-level program at a crap 6th-form school here, and she's just finished her A.S. exams. So far she's had two provisional offers, from Durham and Leeds, and one flat-out rejection, from Birmingham. She's still waiting to hear from three other universities. Cambridge and Oxford were not even an option with her American background (stong but irrelevant) and scant A.P. dossier. I'm not sure she would have been comfortable at Oxbridge. My husband went to Cambridge and certainly his humble background did not prepare him for a lot of the social aspects of the school. It's hard knowing we've limited her options by making this move at this critical point, but she wouldn't have wanted it any other way.
Is your son doing an A-level course, or did he graduate from an American school? Is he being judged on his A.P. results? And has he heard from Warwick or St. Andrews? I was visiting a friend in St. Andrews two weeks ago, and what a fabulous place that is!
It sounds like my daughter and your son are going through the same process. I'm interested to know what you and he decide in the end. Good luck!