I'm wondering if he is getting treatment for his depression. Depression can lift on its own or get better with exercise such as walking, but he might need to see a therapist and a psychiatrist if his depression lingering and even getting worse.

Do you have any inkling why he might be depressed? For example, has he suffered a lot of losses lately, is he struggling with a health issue, etc. Or does he have clinical depression evidently related to brain chemical imbalance?

I'm wondering because the causes might indicate what he needs to get better.

You are a dear to stand by him, but he needs to be trying to help himself get better, too.