Good morning and thanks for your question.
It's clear from your description that your son demonstrates some exceptional talent(s). I'm glad that you reward him for his effort (not just his accomplishments) and clearly want the best for him.
I also appreciate that you are writing with your initial questions to a mental health forum here at JA. It would appear that your son does not have any medical reasons for his declining performance (based upon what you indicate), but it might be good to be 100% certain that there are no physiological reasons behind this recent change.
Once you feel comfortable that there are no medical/physical reasons for this change in performance - I would encourage you to seek out the advice of a specialist in this area. That is, a licensed mental health professional in the area of SPORTS MEDICINE likely seems a much better source of information and intervention than a traditional Pediatric Psychologist or Psychiatrist. At least, if you had brought your son to see me, that's what I would recommend.
(Simply put, there appear to be some performance anxiety issues at foot here. Working through that anxiety with a licensed mental health professional who KNOWS SPORTS and KNOWS ATHLETES makes much more sense than one who does not.)
I regret that, being a Yank, I can't refer you to someone there in Britain with whom I've worked. I *can* suggest you explore: http://www.sportspsychologist.co.uk/ or http://www.bps.org.uk/careers/what-do-psychologists-do/areas/sport.cfm or even Stephen Bull's book "Sport Psychology: A Self-help Guide" as a first step. Your son's coach might also know of some... or possibly at the local children's hospital or university.
In short, your report suggests that your son has exceptional talent. Many people with exceptional talents also have exceptional needs to keep those talents top notch. I would encourage you to consider working with a professional with unique knowledge into the mind and body of an elite athlete. A Sports Psychologist would be an ideal choice.
Thanks. I hope this was helpful.
* FEEDBACK ENCOURAGED. Please contact me prior to leaving negative feedback so that we can resolve the matter. I am eager to work with all JA clients to provide them with useful/helpful answers. Thanks again.