Could you have gotten it back after he passed away? Or was the kidney forwarded on to someone else?

Not sure why your question is being downvoted. I think it would be fascinating to read about a kidney making a journey from son to father to son.

You can't get it back there's nothing to attach it to. There's also no point for the risks involved.

Organs can be transplanted twice but it's very rare and it's usually when the first recipient dies shortly after the transplant. It's called (quite obviously) retransplantation if you want to Google it.

I don't know why you're being downvoted, because this is an interesting question to me at least