Thanks for the reply.
Individuals can't directly access their own 'criminal record' (i.e. the data on the CRB computer). However, everyone has the right to access their 'police record' (i.e. the data on the Police National Computer), which is basically the same.
To request a copy of the relevant information, your son should write to the police force covering his home area. Although the application can be solely by letter, it's far better to use the form which the police force can provide. (i.e. the initial letter should simply request an application form. Using that form will ensure that your son answers all of the relevant questions).
The applicant has to submit proof of identity with the form, as well as a fee of �10. As an example, here's the form used by Surrey Police:
http://www.surrey.police.uk/DP_Access_Form.pdf
Chris