Seeking some medical expertise!

[Sydnie]

Yeah, I've been thinking about it some more while trying to work through the chapter, and I'm starting to feel like I should just go with something not necessarily magical but fictional. Really, the most important thing is just to have him suffer from something that can't be cured, only treated, and losing access to the medicine leads in a very direct sense to death. As long as I can take Tobias away and establish why he couldn't get the medicine anymore, I guess it's fine if the details are a little, let's say "fabricated". I'm just on the fence about it because I was really attached to the idea of directly referencing an actual condition for that extra bit of oomph for savvy readers~

 

[Eddric]

I'd still say diabetes is a good way to go with what you described. I've been in the medical field for about 10 years at this point, EMT to nurse, so I've seen a lot of the problems that comes from rationing or not taking the prescribed medicine. Especially over here in the US where pharm and insurance company practices are downright asinine, spiking prices on very much needed medicine that only costs like $1.50 to make, if that.
But yeah, either diabetes type 1 or 2 could work. Even though, yes, technically one can reverse the effects of type 2 with diet, and exercise, that doesn't happen in days, weeks or even months. The character can fall into a diabetic coma or get DKA or something. Hyperglycemia will kill you long, hypoglycemia will kill you fast. Neither is pretty.
If you'd prefer something different, you can also go with Addison's, Wilson's, or Myasthenia Gravis. Those can have some weird symptoms, some more obvious than others, but may not be immediately knowable to readers.

Edit: Maybe Lupus?