That's actually much too small to be a sand worm of Dune. A baby one, to be sure. The sand worms of Arrakis can eat a harvester in one gulp. A harvester has a crew of almost 30. That thing's mouth isn't large enough for two men. Much less thirty and the giant machine they rode in on.
I think it's likely that the Fremen spent hundreds of thousands of years riding the worms, to the point where they might have shared some sort of psychic link with the worms. The book is very clear that psychic potential exists within the setting, so who's to say that the Fremen don't have some sort of mental link with the worms? If that were the case, the hooks would merely be for holding onto the worm, or possibly a way for the worm to know one of the Fremen were there.
As Uber says, the actual method of maker-riding was left pretty vague. There are many possible explanations as to how the riding is possible, though few of the plausible ones directly involve the hooks as anything but ways of hanging onto the worm.
Yeah, that worm looks a bit small to be able to support his weight. A baby one, the ones used for creating the Water of Life, can be held down rather easily by just two people, so I doubt a worm that small could support his weight.
Judging by the date of your post, I'm afraid I'm too late to stop you.
If you have any plans for reading any more, stop. Don't. Leave it alone at the first book. The second is awful, the third is worse, the fourth is worse than the third, and it continues down this pattern until they finally end.
Though the prequel books are pretty good, so far as I've read.
I only read the first book. It's just not Dune anymore if it's not on the desert planet of Arrakis, rather than a lush forest planet that happens to be of the same name. Without sand, what would happen to the sandworms?
Thing is, it's not even possible for a desert planet to be converted into a lush forest world. It's just not. It would have been better if there were patches of forest among a planet that was still mostly desert.
I just doubt a desert planet could become mostly lush. I'd say it would gain a water table and some patches of lush forest oases, but would remain desert for the most part. You couldn't make that much lush forest without offworld assistance, and I believe they didn't have offworld assistance for that.