my parents were, and still are exactly the same!
i know its hard, but what you need to do, is get him to look up depression, especially major depression
direct him to these sites
beyondblue
Clinical depression - Wikipedia, the free encyclopedia
and any others explaining the types of depression
he will then see, that some depression isnt about whats right and wrong
sometimes its just your brain having chemical imbalances and stuff like that
and everyone has reason to be sad, even tho you dont need reasons to be depressed, sometimes you just are
life is such a bittersweet thing, theres always some bad.