First off, he's got copyright protection as soon as the works are created. REGISTRATION gives you some additional features.
He may have issues with this however. If the works were ever disseminated to the public as individual works, then they can't be registered in a collection. Here's the operative text from the law (you should definitely by the way read Copyright Circular 1 before you even think about doing registration yourself):
Under the following conditions, a work may be registered in unpublished form as a “collection,” with one application form and one fee:
• The elements of the collection are assembled in an orderly form;
• The combined elements bear a single title identifying the collection as a whole;
• The copyright claimant in all the elements and in the collection as a whole is the same; and
• All the elements are by the same author,