A Dependency Discharge generally relates to the active serviceman being the sole and only person available in a family to take care of an immediate family menber--a parent, sibling or child.
Box 40 could even relate to you (?).
The language permitting his discharge under these circumstance is contained in the (AR) Army Regulation specified which was codified on the date give--which applied as late as the date he put in for the dicharge.
You will likely find the answer for this in your family rather than through official militarty sources.