Dad's week has anchors. It might be a card game on Tuesday, the barber every third Thursday, Sunday mass, a bowling league, or coffee with two men he has known since the old job. When one of those stops, it rarely gets announced. More often, someone else notices first: a friend mentions he hasn't come by, or the barber says he hasn't seen him in a while.
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How the drop-off usually happens
Dropping out tends to happen in stages. He skips one outing and says he's tired. The next week he has a reason that sounds fine. Then the invitations stop arriving, and the group quietly adjusts without him. Often the first visible sign is not a refusal at all but a ride that falls through or a phone call that never gets returned.
Write the routine down
Make a short list of every standing commitment he has had over the last few months. For each one, write who runs it, where it meets, how often, how he gets there, and who drives. Then mark which ones he still attends, which he talks about, and which you have only heard about secondhand. The gaps on that page are your starting point, and they are much easier to argue about than a vague feeling that he's pulling back.
Ask the people who see him
Call the one person who sees him most often outside the house. Ask three specific questions: when did he last come, did he say why, and did anyone ask him to come back. Keep the call to a status check, not a gossip session. If the answer is that nobody has asked him in a month, that tells you something about the group as much as about him.
Separate the likely causes
A pullback usually has one of three sources, and each needs a different response. It can be fatigue or weather, which often eases on its own. It can be fear of going out after a stumble, which you can address by offering a steady arm or a ride with a fixed drop-off point. It can be a change in how he gets there, such as giving up driving, which deserves its own plan. He may also stop because he doesn't want to slow the group down or because he is embarrassed about hearing or balance. Listen to his reasons before you test them.
Know when it is a medical question
If the drop-off is sudden, or it comes with confusion, changes in sleep or appetite, or new trouble with words or memory, that belongs with his doctor. You don't need to diagnose anything yourself. Name what you saw, when you saw it, and ask for an appointment to look at it.
Next move
Tonight, call the one person who sees him most outside the house and ask the three questions above. Then send Dad a specific invitation to one outing this week, with a fixed time and an offer to drive. Note whether he says yes, and whether he shows up. If you want the wider setup for tracking this kind of change, start with Start Here.