He used to trade in his cars after retirement every two years. Reasonable. Loves cars. But now it is becoming obsessive. Car dealer makes out really well with each trade in and up for new model and highest in their lines. Last one had 2500miles only. that was 7 wks ago and now he is working on another trade. He sees nothing "wrong" with it and the dealer will not stop him.
He's probably dissatisfied with his self-image, and since he can't do much about the way he looks perhaps changing cars every so often makes him feel better about himself and in control.
I suggest you get it straight from the horse's mouth. Ask him how he feels before, during, and after trading an "old" car for a new one. ... Pay careful attention to his body language, and observe/monitor him often -- especially when's he's not aware -- so you can anticipate his behaviors.
Still, I have one question to ask: If he were in his 30s or 40s, would you consider his current behavior odd?
If you think the dealership is taking advantage, you may want to intercede. However, making a commission is how dealers earn a living. They are not going to turn him away.
Your father may feel that he is nearing the end of his life....why not enjoy the fruits of his labor?