A good library will have a contempory issue of "Who's Who in America" or maybe "Who's Who in American Business". The reference librarian will get you the right volume. If your great uncle held a leadership position in a major US corporation, he would be listed. You could also get his obituary in the local paper serving the town that he lived in when he died.
CEO and President are usually interchangeable terms. Chairman of the Board is usually a different position.