A standard level CRB check only shows convictions and cautions. So your son's arrest won't show up on such a check.
An enhanced level CRB check (which is carried out when someone applies to work, for example, with children or vulnerable adults) shows convictions, arrests and any other information which a senior police officer (or other relevant body, such as Social Services or the Department for Children, Schools and Families) deems to be relevant.
Whether your son's arrest would show up on an enhanced CRB check would depend upon whether the police were convinced that he was innocent (in which case it shouldn't) or whether they simply thought that there was insufficient evidence for a successful prosecution (in which case it almost certainly would).
Following the Soham murders, the police are now under considerable pressure to ensure that enhanced CRB checks show all relevant information, including all unsubstantiated allegations of a sexual offence. (Ian Huntley had been investigated for alleged sexual misconduct, on several occasions, before he moved to Cambridgeshire. The enquiry that followed his convictions for murder strongly criticised the police for not including these matters in his CRB check). There was a post, earlier this year here on AB, where a guy was totally unaware that any allegations had ever been made against him until a CRB check was carried out. It was only when he read the report that he saw that he'd been suspected of 'grooming' a child for sex.
Chris