At a minimum, your grandfather’s BRLS record should be available through Ancestry, search Military. The BRLS usually has a bit of information. If he was Navy or Marine Corps there should also be muster rolls under that search.
There were certain categories of records for the army and air force that were destroyed, by date and by last name. There are web sites where you can request those records from DoD, even for deceased veterans. They have some records going back to the 1700s.
I will try that. He was Army.