Have you tried requesting a copy of his military service record? A record that old would have every action included in it. We requested a copy of my dad's service record from the Army and were able to find our half-brother that my dad had no idea where to look for. (There was a monthly allotment at one time for his son's support.) With the address, we were able to trace forward. Considering the name change, maybe it'll give you a lead!