Depression is a mental state, so no point keeping a strict check on his physical health. As you said, he has been a workaholic, so being at home and doing notihng might be killing him. It may have led to loss of confidence and enthusiasm, or probably he might be thinking about the home life that he missed during all those years and is unable to connect with now. He might be trying too hard to bridge the gaps, but his low confidence and may be guilt is making him try too hard without any progress.

You'll have to be patient. Show that you understand..try to communicate that he doesn't have to try hard and his family is there for him.

Here is an out of the box idea,sure to work but somewhat weird to follow. But if you are intent to change the state of affair of your life ,well give it a controlled trial.Introduce him to a emancipated group of ladies and have frequent outings and mix ups.let him have a soft extramarital relationship and see how his depression evaporates into thin air.Mind you,exceptional situations ,demand exceptional solutions.

'Men will be men' therapy! Nothing exceptional about it :P

Divert his attention, where his interest lies... Like playing a video game / chess. Take him for walks in the evening and make him friend with other people. That intraction will help him to come out of depression. Don;t do anything for him for dutysake. Do it with love and express your love for him in every action. And make him understand that his depression is making life hell.

Let him write articles for magazines and websites....