With Admiral, we added one car to a multi car policy (it was at that time the only car on the policy) and when the second vehicle came up for renewal it was added. My wife and I were each the main driver on one vehicle and the named driver on the other hence we both kept a NCD.
He might want to check with the insurance ombudsman because I'm sure there is case history whereby you are only required to notify them at renewal of minor speeding convictions or changes that would have a bearing on the renewal (although I don't think that would apply to vehicle modifications)...