I have a 11 year and had te same problem. He stopped but we were constantly complaining about it. I put clorox wipes next to the toilet and kept making him clean it up. He eventually stopped but it was frustrating.
Does happen all the time or just overnight? If the latter, not much you can do I don't think. My brothers would get up in the middle of the night to use the restroom and not even remember it. They were still pretty much asleep.
Day time? I wouldn't hound him. I'd remind him that he needs to lift the seat and then have him clean it when he doesn't.