Persistence is a virtue when pursuing genealogy records!!!
I'd like to suggest you get a certified copy of his death certificate to submit with your application.I believe if you can provide this along with your application, you may be able to request information under the Freedom of Information Act (FOIA).I would also suggest you show how you are related if the form requests it.
I believe once a person dies and you provide proof of death, the records are open to the public under FOIA.Have you checked this out?
There may also be reasons why you aren't able to gain access.Have you asked them if his records are sealed for any particular reason(s)?This happens if part of the documents are classified and haven't been unclassified.The rules related to classification has changed over the years and if they are classified, you may want to request declassification.