This looks to be his enlistment record at the NARA, it has his Army Service Number and you should help in identifying his IDPF if there is one. Also be able to order a copy of his service record if you ever find a death certificate to submit.
http://aad.archives.gov/aad/record-detail.jsp?dt=893&mtc...He may have began to use his middle name. Having an exact birth date would also help.
Or he starting using his Step-fathers last name Hildebrand.