Most likely your son has schizophrenia, which often manifests during the teen years.
Since you live in the UK, your son should be evaluated and treated by a community mental health team (CMHT) or an early intervention team. I believe your GP is the person to refer him there.
You can read more about schizophrenia and care here http://www.nhs.uk/Conditions/Schizophrenia/Pages/Treatment.aspx
I would call your GP and urge that he/she refer your son. There is good treatment for schizophrenia today, but it's not easy and requires help from a number of resources, as outlined on the website I listed. Best wishes to you and your son.