It stays on the Police National Computer for life and therefore will show up on any CRB check. After a certain period of time you can treat it as spent, that is, you don't have to declare it and an employer can't use it against you.
However, some occupations are exempt from the Rehabilitation of Offenders Act and in these cases the conviction is never spent and therefore you he will not be able to be employed in certain jobs
Hi any conviction will remain on the ACRO police live trace system for 10 years. After this time it will be stepped down off the life system but always on his record dormant. If he requires police clearence in the future for jobs, visa etc it will appear for the next 10 years purely because he was convicted. Hope this helps