An 11 year old simply cannot receive a police caution - it would have been a 'reprimand' for a first offence, or a 'final warning' for a second offence.
Usually the Youth Offending Team get involved, and certainly the parent would have been contacted.
In any case, a reprimand or final warning is not a criminal record, and the police record of it ends after 5 years or on his 18th birthday, whichever comes first.
From what you have said, I would be extremely surprised if your son has received a reprimand. Tell him to forget all about it.
More info here:
http://www.direct.gov.uk/en/YoungPeople/CrimeA ndJustice/CrimeAndTheLaw/DG_10028367