Obviously working off the same information as you, but it sounds more like he stopped him from leaving during the winter, and didn’t stop him after. Kitty made his choices and regrets them now.
You don't let a cat outside when it wants to go outside, their instincts tell them they need to survey everything they can see, my cat is EAGER to run out the door and then he gets outside and is terrified and wants to go hide. Not every cat is terrified but once you have an indoor cat, it's an INDOOR cat.
I met my “timeshare” cat cuz the grump was stealing carrots and got stuck in the greenhouse. Fond memories there. Still don’t know what he did with them. I like to think he brought them to some other thoroughly confused human as tribute.
Maybe you can just establish communication with his other family so they know he's visiting you and not hit by a car. Sometimes a noisy bunch of kids can cause a cat to run away, even though they love him.