I don't know if any of the leather condioners/protectants will prevent ink stains, but I can offer what might be some encouragement.
The back of the seat is probably vinyl.
So what, right? It's still a stain.
Yes, but a mark from a Sharpie pen that I used a lot of products on, (some pretty aggressive, lacquer thinner for instance) disappeared on its own from the vinyl dash of my previous Pontiac mini-van. I had given up on removing it and one day I noticed it was gone. Maybe the ink doesn't penetrate the vinyl and just disappears after a time. Hopefully that will happen with your ink mark.
Oh, and keep the kid. If that's the biggest problem you ever have with him, you will be fortunate.
Charles
