Forget about 80% to 95% of all this "good advice" (read: bull****) that's so freely dispensed. Assuming your cousin IS, in fact, a Viet Nam Vet and IS, in fact, disabled as a result of exposure to Agent Orange or otherwise entitled to compensation, have him contact Disabled American Veterans (DAV) or American Legion. Have him inform them he wants to FILE A CLAIM and ASSIGN THEM POWER OF ATTORNEY TO HANDLE IT. YOU DON'T NEED RECORDS. THEY CAN LOCATE AND ACCESS ALL THE INFO THEY NEED TO PURSUE THE CLAIM.
If and when he's awarded compensation, DVA will then take over and do THEIR thing, which is primarily disbursing checks and administering medical care.