His friends might be able to save him. If they notice his disappearance quickly enough, they could band together and search for him. Maybe they have some knowledge or items that can be used against the demon.
Dallas could be a great addition. He has that tough exterior and his relationship with Johnny and Ponyboy is complex. His presence could add some drama and conflict. Another good character could be Sodapop. He's Ponyboy's brother and his kind and fun - loving nature could balance out the more intense moments between Johnny and Ponyboy.
In this fanfiction, it might be due to his poor living conditions. He doesn't always have the best food or a warm, clean place to stay. His body could be weakened from not getting enough nutrition, which makes him more likely to get sick. Also, the emotional turmoil he goes through with losing friends and dealing with the Socs could take a toll on his immune system.
The story concludes with Ponyboy having a changed perspective. He overcomes his difficulties and gains a deeper appreciation for the people around him. This leads him to a more positive and determined future.
The other characters' reactions could vary. Some of the more hot - headed gang members might initially be impatient about Ponyboy being sick and not being able to participate in their activities. But as they see how serious it is, they would come around. The Socs, if they knew, might not care much on the surface, but there could be a few who feel a bit of sympathy, especially if they start to see Ponyboy as more than just an 'outsider'. And the girls in the story, if there are any, would probably be very motherly and bring things like soup and warm blankets for Ponyboy.