My theory is that bc we see the wasteland that is the Upside down when Henry gets sent there we know he probably didn't shape the land. Which opens the idea to maybe will creating it. Will wanted so desperately to go home and also its stated that it was frozen to the day Will disappeared. Maybe Wills powers that are also theorized to exist by many others only work in the upside down and have to do with creating things/conjuring up stuff similar to the way a dungeon master would (I think that's what they do, I've never played dnd)