Some Help with Druids

Hey guys, first post here. I ran into one of the authors of Dungeon World on Reddit and he directed me here if I had any extra questions, and so here I am.

I ran a Dungeon World one-shot to get used to the system (using the free sample module Indigo Galleon), and that got me roughly on the right footing for playing the game properly, so I started running an actual campaign for 3 people who hadn't played before - of course my experience is only a few hours greater than theirs in this system so we're learning together. One of them is a druid and I have some questions about what his shapeshift power can truly do for him.

Some necessary background: the world we created together while making characters was a sort of combination of Pirates of the Carribean and Mad Max - a relatively lawless wild west ruled by warlords with access to landships to aid in crossing the hostile terrain. The world is by necessity relatively harsh and includes features like sandworms (a la tremors or dune or even of the infamous purple variety), wyverns, all that good stuff.

The Druid, being a smart lad, chose for his domain "The Blasted Wasteland", which correlates closely with the terrain on which the campaign mainly operates. This presents a slight issue for me in terms of the scale of power the Druid may have, since the sorts of animals that inhabit this region - if they can be called animals - includes stuff like giant purple worms, wyverns, etc., and turning into such incredibly fearsome creatures seems almost like cheating, which to me indicates that I'm probably doing something wrong.

So, where is the line supposed to be drawn on Shapechange? Can a Druid turn into a dragon and bellow forth great gouts of flame and whatever? Doesn't that seem a bit... I dunno... strong? Should such fearsome monsters be considered "animals" at all? Or is there some sort of natural consequence that I could use to temper such potent abilities that seem to allow overcoming of obstacles with nearly contemptuous ease? What do?

This sort of question you should ask the player or the players.

So, Druid is all about natural, right? Druid, what's natural in the wasteland and what is not? Wizard, what foul beasts out here are your kind's creation? Druid, do this worms actually have a spirit?

That would be a cool way to manage the former solution I thought of ('no you can't be a giant sandworm because that's not a natural animal'), but if the players don't cooperate along that line I'm still stuck with a guy who can turn into a giant purple (sand)worm, which still leaves me feeling out of my element. What should I do if that happens is more the question I'm concerned with - should I just not let it happen, or is there something I can do (I sadly haven't thought of such a thing, which is why I pose the question) to help deal with a guy who can turn into a dragon or giant purple worm so that he doesn't run away with the game?

So it is now established in fiction that he can turn into a giant worm? I wouldn't do anything about it then. 
You don't like him turning into giant worm? Then tell the Druid: worm's spirit is greedy and corrupted, he will try to eat you inside. You can handle it for now but it's dangerous. What do you do?

I agree with Vsh, his player questions are great. I don't think it's too late to just discuss your issue with the player and retcon it, but if you think it's too late then corruption seems like a neat side effect. Otherwise, definitely discuss it. Like, maybe he can only shapeshift into a creature of similar mass. Or maybe it has to be "natural." It's always fun to talk about what the move means at your table.

Remember, the druid doesn't just get the strengths of the animal they turn into. They get all the weaknesses too. Look for vulnerabilities that the druid's enemies can exploit.

He's turning into a giant worm? OK, cool. Show him a downside. Are there are side effects caused by the worm's insatiable appetite? Is turning into such a huge creature physically draining? Does it's absolutely massive size make it impossible to dodge attacks? Does it's soft body mean it takes extra damage? Is there a group of people in the world dedicated to hunting down and exterminating giant worms? If so, do they notice that there's a man wandering the wastelands who can turn into one of them at will?

That would be a cool way to manage the former solution I thought of ('no you can't be a giant sandworm because that's not a natural animal'), but if the players don't cooperate along that line I'm still stuck with a guy who can turn into a giant purple (sand)worm, which still leaves me feeling out of my element. What should I do if that happens is more the question I'm concerned with - should I just not let it happen, or is there something I can do (I sadly haven't thought of such a thing, which is why I pose the question) to help deal with a guy who can turn into a dragon or giant purple worm so that he doesn't run away with the game?

What are the sandworm's moves?

First a thanks to all of the potential consequences if that was part of the fiction - I was having a hard time thinking of that stuff myself, and it makes me feel better that if we decide that's part of the fiction that I'll be better prepared to deal with it.

Secondly, @noclue I'd say that such a creature's moves would be along the lines of Burrow Through Earth and Swallow Whole, which coincidentally, giving it a look, are basically identical to the moves the actual Purple Worm is assigned in the Dungeon World book.

Those sound like fine moves. So, the druid turns into a purple worm. He's holding 3 forward. His moves are Burrow through the Earth, Swallow Whole, and Poison Sting anyone attacking from behind. Those look cool.

Let's say they're in a fight with some goblins. The Druid swallows one whole. You rule that a measly goblin isn't going to be able to hack it's way out of a Purple Worm. Dead Goblin. That's one move down.

Another Goblin comes at the worm from behind, keeping away from that tentacled maw. The Druid stings him dead. Second move down.

You reach into you're bag of GM tricks and pull out Show them a downside of their class or race...Well, the worm is really big. It's having trouble maneuvering in the cramped corridor, but the goblins are scurrying all over the walls, swarming it. Grim Portent. You ask what the Druid does and he wants to burrow out of there. He's got no trouble burrowing, but you call for a Defy Danger since there's goblins jumping all over him. Can he get out in time, before they start hacking into him?

He rolls a 10+, burrows out of there and into the corridor ahead about 100 yards. That's 3 moves, Druid isn't a purple worm no more. You grab your GM move of separate them, since he's down the corridor now...

Also, there's this post by sage

The druid turns into a purple worm and swallows a couple of goblins. Now he's got some monsters in his wormy gullet.

So what happens to those goblins in his tummy when he turns back into a human?

Could get messy...