It sounds like you are describing the source of his depression as being his relationship to the memory and impact of what was, for him, relatively too much time

It sounds like you are describing the source of his depression as being his relationship to the memory and impact of what was, for him, relatively too much time working throughout his life. While the history here cannot be changed, the way he is relating to it can be changed. This is not necessarily very easy to do, but for most humans, a distinct possibility, with the right tools and resources. The source may be able to be de-potentiated, the emotional charge these memories carry in his system may be unwound, which can have a great effect on his quality of life.