Sandworms are a metamorphosis of groups of sandtrout surrounding and consuming pockets of water. Humans have a good bit of water in them, they can sense it easily in the desert. So he lets himself get ‘consumed’ by sandtrout, using his bene jesserit skills to force his bodies composition to accept it as a symbiosis. It starts out as basically an armored exoskeleton with the durability of a sandworm, and he grows into a much more sandwormy being.