He is not eating. You noticed before you could name it, in the way the pot is still full on the stove and the bread has gone hard in the bag. The question is whether this is a hard stretch or the start of a pattern, and the answer changes what you do tomorrow. You do not need a diagnosis to start looking. You need a few things you can see from the kitchen door.
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Look at What Left the Fridge
Open the fridge without commenting on it. Notice what has been opened, what has been thrown out, and what is still sitting where it was. A carton of milk that was full one morning and half the next is information. A fridge full of sealed leftovers is information too. Watch the trend, not a single shelf. Are the things he used to like going down, or sitting untouched?
Count the Plates and the Trips
The dish rack, the trash, and the pantry tell you more than he will. Count how many plates or bowls were used in a day. See whether he has been to the store, whether anything has been ordered, and whether the bread and cereal boxes are nearly empty. A man who has stopped cooking for himself may still open a can of soup and eat half. Track it for three or four days in a notebook so the pattern is yours to look at, not just a feeling.
Listen to What He Says About Food
Pay attention to the sentences he reaches for. I am not hungry. I had something at lunch. I will get to it later. The church ladies brought too much. These are common phrases from people who are tired and grieving, and they often mean the kitchen is hard to be in. Do not argue the point. Try a small, specific offer instead: a sandwich cut in half, the soup he liked, a plate you sit down and eat beside him.
Look at the Rest of the Day
Skipped meals rarely come alone. Check whether he is sleeping in the chair, whether the mail is piling up, whether he has showered in the last few days, and whether he has stopped going out to the places he used to. Write down anything you see. Together these make a clearer picture than any one of them does.
When to Ask the Doctor
Call his doctor if his clothes fit much looser than before, if he seems confused about meals or the day, if he is drinking very little, if he has trouble swallowing or keeping food down, if he is dizzy when he stands, or if he has fainted. Bring your notes. A list of what you saw and over how many days is the most useful thing you can give the doctor.
What to Do Tonight
Tonight, do one small thing. Put a food he likes in the place he usually sits, at the time he usually eats, and sit with him while he has it, even if you do not eat yourself. Keep the rest of the week simple. If you are reading this in the middle of the night and need the wider plan, start with Start Here: The First Hundred Days, which walks through the first weeks in order.