I'm fairly sure that if its a UK ATPL then the PPL priveleges will be non-expiring. As the medical is valid (1 year for class 2 privileges) it is then only a matter of getting a SEP (land) class rating by passing a test with an FE or CRE.
Unfortunately no homebuilt aircraft in the UK are authorised for night or IMC so he would be limited by permit to fly limitations rather than his license privileges. He could however exercise IMC privileges in a suitable CofA aeroplane.