Bonnie, Ancestry has a Veterans BIRLS index file of deathsthe VA benefits were used. It has name and service dates. A Fold3 genealogy site also has military databases. Both are subscription and your local library may have one to use for free. A local LDS Family History Center may also. If you know your brothers birth date as well as the city he lived you may still use the SSDI to check. Also Ancestry has a Obituary database of some newspapers as well as a public records database you can look up his address only ans see all who are listed as living there.
There is a Veteran's National Cemetery database.. www.gravelocator.cem.va.gov
Your best bet if you don't mind really making a son in law unhappy is to call the adult protective services in the city or county he lives in and ask to do a check of his welfare. You may even contact SSA to ask them also to check to see if he is still alive. If you question about his death they may request him to appear in an local SSA office to prove he is alive to still receive benefits. If he doesn't appear they will stop benefits until he does.
I subscribe to Ancestry so if you send me a email directly I wil do some searching for you. Need his full name and address and date of birth.