I doubt a small rat once a week for a 1300g ball python would cause fatty liver in itself. They're still growing pretty steadily at that size, except during breeding and fasting periods of course.

I'd personally attach a note to any of his offspring that I put on the market so prospective buyers could make a fully informed decision. You're just asking for bad press otherwise if you sell a bunch of animals and this ends up being a genetic issue.

Maybe contact the person who produced him and ask if they've had similar problems from that particular line? What kind of feeding routine were you following while growing him out?

Whatever the cause, sorry for your loss.