In the book it was explained that he could use his own excrement because his bacteria was the only bacteria in the environment. That meant that his body had already created defenses against it.
I found the science in the book to be logical, explained, and plausible. There was no dues ex machina to come save the day.
If they stuck to the book, it was “doable.” The book took place over a couple years, so the timeline is probably compressed.
“The book took place over a couple years, so the timeline is probably compressed.”
In the movie he was alone for ~550(?) days...