If he never registered it in his name, then the last registration would have the previous owner listed on the FAA website and his address. Without a Bill of Sale from the previous owner, I think you're stuck with the FAA until his family get's their hands on the original or a replacement from the last registered owner.
If the airplane has NEVER been registered, only an N number reserved, then you still need a Bill of Sale, which the "original" one comes from Vans Aircraft that they issue to the kit serial number when you are ready for the FIRST actual registration with the FAA.
I'm a little confused on where the airplane is in that regard.