Joan-- Without the benefit of the particulars of your relatives loss, perhaps some general comments will do.
Depending on whether your relative was Army or Navy, the ongoing status of the efforts of the military to locate him would be contained in what is called an IDPF.The IDPF is a File 293 Individual Deceased Personnel File.
This file will contain all paperwork relating to his loss and the investigation to determine his status.If he was in the aviation branch it would contain a copy of something called a MACR (Missing Air Crew Report)which would give the particulars of his flight and the people aboard his plane.If in the ground forces, the IDPF might contain a tactical summary of the movements of his unit during the time of his loss. If he was aboard ship it would detail the ships movements and last position.The IDPF attempts to create a complete record of the missing person--sometimes it is very detailed (and graphic) and sometimes it is not.These files have been used extensively over the last 50 years and occasionally one finds that they are non-existant or incomplete, but they are a major starting point on an investigation.Based on the information available the military must determine the final status of the MIA, and in order to begin the benefits process to the next-of-kin they established the year-and-a-day plan.
The IDPF is not the end of the process for you.It is only the beginning.You should endeavor to do your own parallel research on his unit and its movements at the time of the loss.There are websites and veterans organizations listed on the web that can help you understand the history surrounding the loss.The folks that hang out here have a diverse knowledge of WW2 history and various specific aspects of the war.If you were to give some particulars of his name, and unit, perhaps we could be of assistance in getting you moving in the right direction.