I'm actually pretty prone to suggestion in regards to my obsessions. I got obsessed with baking and cooking in general because of Good Eats, I got obsessed with poker from watching the WSoP, I got obsessed with nutrition from an argument on another forum... but I don't think most autistics are like that.
As for turning him from the sugar... I'm not sure. The advice I'd give in most cases doesn't really fit on a primal forum (boiling broccoli, for example, is a terrible way to cook it, but primal eaters know that).
I guess the only thing of which I can think would be a slow transition, much like watering down fruit juice more and more every week until it's water. If there are any other foods in which he's interested, try incorporating them into dishes.
I'm sorry, but that's all of which I can think. I'm a pretty picky eater myself, which was one of my major challenges when going primal.