92 y/o First Claim

Hi. Hopefully this is the right group. I’m trying to help my grandfather file a claim. He's 92 y/o, housebound, and under my care. The DAV doesn't have appointments until late next month. His records were likely destroyed in the VA fire because the archive agency hasn't been able to find anything (not yet at least). All I have is his DD214 and private medical records. He served in Ft. Sill, Ft. Leonard Wood, and Ft. Chaffie from 07/25/1952-06/15/1954 and ??/03/1954-01/25/1956 (Month unknown because DD214 is unclear to me).   I'm unsure how to prove he was exposed to the toxins he is claiming. I'm also not sure if I'm filling out his Claim Paperwork properly.   Any advice or assistance would be appreciated. I filled out the Intent to File and Priority Processing paperwork. Thank you all for your service!