If there were any court actions concerning a declaration of death, I'd start with the court where his wife was living.

Try Googling for the relevant procedures.

Back at that time, it was so easy to disappear that they would just settle somewhere and not even change their name. So it would be worthwhile to search as if he were living. If and when they had him declared dead might not be as useful since there would be no information on his actual date of death or the actual cause of death.