If these signs last more than three weeks, the trouble can be serious. The first thing is to give your full support. Let him know you care. Then get him to a doctor. Also important, keep him busy.
Physical activity is a great help.
But start out with fairly simple things. Otherwise, you may add to the very feelings of failure that you're trying to remove. There are a few things to look for. When the person's appetite returns, when she begins to sleep normally again, when he takes an interest in his family, the person will be getting better. Another sign is her being critical and cranky.