Sorryyy I'm not writing a fic (I never write the fic rip) but anyone totally could??? Feel free to! I still wanna talk about this idea though! So let's say everything remains the same, except Hiccup and Toothless' respective injuries are reversed!
I'll say it is likely going to take Toothless longer to find empathy for Hiccup than it was the other way round, because Toothless has a different, more aloof character. The emotions have got to be more ramped up than the dragon's soundless desperation which originally prompted Hiccup to save Toothless' life.
Hiccup has got to be loud. He's not going out like that.
Would he scream for help? I don't know. Let's say he knows that no one will hear him from the distance of Berk's village. Also, the dragon might hear him and come back to finish the job. So no initial screaming here, just a lot of complaining and talking to himself.
But let's say during the skirmish where Hiccup and Toothless ran into each other and fought... say, Toothless fired a plasma blast and it hit some flammable wood in the cove and it created a coal fire. Hiccup is going to want to use that to cauterize his wound (sorry for being so graphic). And then there's gonna be some screaming. That can draw Toothless back in to check on what the heck happened to that human.
Hiccup may have recovered his knife, mayyybe he is even in possession of his shield. He'll hide in the crevices of some rocks that Toothless can't dig into and build a fort for defense.
He'll have to leave the fort for food and water though. I personally headcanon that Hiccup is actually quite skilled at fishing, the fact just never came up during the movies haha! Still, he'll be sooo slow and in sooo much pain and he's terrified that the dragon will spot him and finally kill him this time around. Toothless, observing the clumsy human's efforts to stay alive, now has the chance to earn his trust by catching fish for him, by ripping out entire bushes of berries (Hiccup'll eat the berries and use the sticks for firewood), by keeping him warm with his fire beams, and providing buckets of water somehow.
I want a scene in the pouring rain where Toothless is shielding Hiccup with his wings.
...Well, you know the deal, they become the unlikeliest of friends and then, at some point, Toothless has to decide to return Hiccup to his village, walking into a certain death trap for himself. Yep. Very yep.
I know there are stories like this out there but I want it with them.