We can't predict, but progression is often fairly linear during the "middle" stages. You would be a better judge if you graph things from before he was diagnosed, using months as the unit.
Having a rollator stashed doesn't require his consent and I would just do it. Order or borrow one based on his height. He will have to be measured much more thoroughly for his personal power chair, and would have to buy into a loaner just to use it.
Not sure how much in denial he wants to be, but might say something like, "You seem to be walking more slowly these days. I read on the Forums that falls are really dangerous for people with this disease, so I've ordered a rollator that can help you walk when you need it to."
And based on what you see over time, which you can also discuss via call or e-mail with your clinic OT/PT (if you are sneaky, you could send video), phase in discussions of a manual chair/power chair with the same kind of approach. If he is a slow progressor, you could also discuss a scooter option. The most important thing is to give him as much perceived control as possible, because ALS takes away quite a bit.