Hi,
I self-studied my GCSEs at home in year 11, and was planning to apply to Cambridge at the time. I had some questions, and wrote to the individual colleges that I was interested in - they all replied and were very friendly and constructive. I would recommend that your son does the same, and I also advise him to do some independent reading around the subject, to demonstrate his motivation - this will stand him in good stead.
Taking a year out of school to study at home was one of the best decisions I ever made, and I actually ended up learning far more than I ever did at school, as it gave me the opportunity to delve much deeper into my subjects and also to pursue my own interests. I know several homeschoolers who have gone on to be remarkably successful. Very best of luck!