I don't know what type of benefits you're attempting to secure, but here's some general information. If memory serves, I believe Hostile Fire Pay was initiated by the Department of Defense in April of 1968 for those stationed north of the Imjin River.
I'm unsure if the person was a single enlistment or career soldier. His discharge documents(DD-214) should show service on the DMZ. Also his pay records should show award of Hostile Fire Pay. Also his army form 2-1 should have notations on his service on the DMZ. Any Leave and Earnings Statements would have the pay included.
You can submit a written request for any records and information directly to U.S. Army Human Resources Command ATTN: AHRC- PDO-PA, 200 Stoval Street Alexandria, Virginia 22332.
If you're trying to secure back pay for the Hostile Fire Pay, you'll probably have to go through the Army Board for Correction of Military Records.
One thing I found was this notation- "As a result of the sudden deterioration of the cease-fire, coupled with the fact that US soldiers were once again fighting along the DMZ, the commander in chief of US Pacific Forces requested that the area north of the Imjin River and south of the DMZ be designated as a hostile fire zone. The Joint Chiefs of Staff supported the request which was approved by the Department of Defense."