He looks too old to have served in WW2, so assuming his military service was restricted to WW1 and before, you could order his records from the MoD. They charge you a £30 admin fee, assuming it hasn't gone up since I used the service in 2015.
WW1 records are very, very patch, however -- many were lost to fire when the record office was bombed during the Blitz. You'll only get whatever they can find.
WW2 records are still closed.