Well, you might consider having Hiccup find a special place or object that gives him the energy and motivation to keep going. Or, have him learn a new skill or technique that helps him manage his exhaustion better. Maybe his friends come up with a creative solution to help him regain his strength.
You could have other characters come to his aid and help him recharge. Maybe they bring him food or offer words of encouragement. Another option is for Hiccup to discover a new source of strength within himself through a challenging experience.
One way could be to have him get some much-needed rest and recovery time. Maybe he takes a nap or spends a day doing nothing but relaxing.
In the fanfiction, Hiccup's exhaustion might be shown through his physical and mental state when interacting with Astrid. Maybe he's less energetic in their adventures or has more internal conflicts.
One way is to introduce a special cure or remedy that Hiccup discovers during his adventures. You can also explore how his friends and allies support him through this difficult time.
You could focus on showing his gradual change through significant events or interactions with other characters.
One way could be for Hiccup to seek the help of other dragon riders or wise elders who might have knowledge about breaking the curse.
One way could be to introduce a magical cure or an event that breaks the brainwashing. Maybe a key moment from Hiccup's past triggers his return to normal. Also, the support of his dragon could play a crucial role in restoring his mind.
You might make the illness a metaphor for a larger challenge Hiccup needs to overcome within himself. It could lead to personal growth and a deeper understanding of his role in the dragon world.
You could have other characters try to wake him up with various methods, like using magic or seeking help from a wise elder.
Maybe Hiccup could learn some self-defense skills and stand up for himself. Or, he could find an adult he trusts to help him solve the problem. It all depends on how the story unfolds and what kind of tone you want to set.
First off, you might introduce a wise healer character who knows the remedy. Or, you could have Hiccup's friends embark on a dangerous quest to find the cure. Another option is to have Hiccup's inner strength and determination help him fight off the poison gradually.
Well, you might consider having other characters give him warm clothing or blankets. Or perhaps he could use his inventing skills to create a device that generates heat. Maybe there's a hot spring nearby that he finds and takes advantage of.