Car insurance which will let son build up his own NCB whilst being on my policy

Well there are some insurance companies which will allow your son to build up his set of no claims discount (NCD) whilst being a named driver (that is with you as the policy holder and him the additional driver) but there's a catch. He will only be able to use that NCD if he takes out a policy in his own name with the same insurance company. This means that he will not be able to switch insurers.