An enhanced CRB check can show anything which a senior police officer (or other relevant authority) deems to be relevant. Following the enquiry into the Soham murders (which severely criticised the police for failing to include unsubstantiated allegations which had been made against Ian Huntley) the police seem to be including everything possible in such checks.
However enhanced CRB checks should only be available to employers when the employee will be working with, or alongside, children or vulnerable adults (or within the security services or the 'administration of justice'). If your son will be visiting clients within their homes (e.g. as a financial adviser) there MIGHT be a valid reason for carrying out an enhanced check (although that would seem to be far from clear) but if he's only going to be working within an office environment, the employer should not be able to get an enhanced check.
Chris