I'd go into the offseason saying NOTHING about the fumbles. If he can't motivate himself to work on that, then it's never going to get worked on. If they harp on it, they're going to put it in his head, where it's been most of this season, and if he's thinking out there he'll die. If you think about fumbling, you'll fumble.
If I'm the coach, I let him deal with it his way - if he doesn't show improvement next year, then I don't let him see the field. But I sure wouldn't drill it into his head, because that's just going to ruin him as a player.
Frankly, I'd be more inclined to push him into losing the weight he gained this past offseason than to talk to him about fumbling.