My understanding was that as it's a progressive disease, there are milestones in progression that help predict future events. Things tend to shut down in a particular order, and at a certain rate (based on past events and stressors).

One of those future events is losing the ability to breathe. Which is just horrifying to think about.

He's had it for at least a year. The disease gets progressively worse with time, and eventually causes patients to lose the ability to swallow or breathe. Doctors can probably give him a good estimate of time left based on symptom progression.

From wiki: "The average survival from onset to death is two to four years, though this can vary.[5][16] About 10% survive longer than 10 years"

https://en.wikipedia.org/wiki/Amyotrophic_lateral_sclerosis