From my experience, it seems there are dips which may be followed by losses which tend to level out or a dip can be followed by a rather rapid decline. It is impossible to predict. My husband is in his 7th year and decline overall has been slow. But when he has a dip, it always stays at that level until the next dip. Some folks die within a few years and some live with the disease for 20 + years. It's always good to plan ahead. I don't think there's a down side to this.
Blessings

The unknowns ARE really scary, especially soon after diagnosis. I remember very well how lost I was.
Each person’s path is very different which makes it really hard to answer your question.
First concern would be to ensure he’s not suffering a physical ailment; Respiratory infection, urinary tract infection, etc.
An illness can really put him in a rapid downward spiral.
If you think he would qualify, I would suggest looking into hospice care. It’s not just for the last few weeks of life, we have been under hospice care for two years now.
Having a weekly visit from a nurse and talking to a social worker once a month has been really helpful to me. It was getting too difficult to get my wife to doctor appointments when she lost her mobility.
Early on, my wife would decline abruptly and then it seemed we would level off or plateau for a time, this cycle would repeat itself. Each decline was scary until you got accustomed to it. So as soon as you get comfortable in what you are doing, another decline would come along and disrupt your level of peace. I found that with time, my confidence level increased and the decline’s didn’t rattle me as bad as they did early on.
Coming to this site was very helpful to me as well. Reading people’s experiences and even taking notes on occasion really came in handy.
I wish I could be more specific but please don’t hesitate to continue asking questions here. We’ll support you the best we can.

When my sister has had a sudden or significant decline to any degree, it’s usually because she has a urinary tract infection.

I find myself going along fine and then a step down. This step down includes memory problems and physical changes. Such as vision problems, heart problems, fatigue, and confusion. Then I go along fine for weeks or months.

He just had lab work, everything was normal. CT scan this week. I know UT’s often are the cause of problems.
Thanks for your input.