Are you absolutely sure he was U.S. Army (Army Air Corp/AF)? Those were the only branch of service records 80% burned. Navy, Marine and Coast Guard were not stored there in 1973. Belle Isle MI has a Coast Guard Station as does Detroit. Something else is you should contact the MI National Guard if he didn't go overseas then maybe his unit was not Federalized. Records could still be in MI only.
Have you filed a Form 180 request anyway with the NARA? If he wasn't Army then they should find his records. A phone call and one is told about the fire...file the paper work and they have to search and just may find something to help you.