Oh no. I was hoping this would not be the diagnosis.
I can't even remember how long Taz went after he was diagnosed with it. But I do know I was giving him sub-q fluids for about two weeks. They didn't work very well for him, obviously. But I see here on PT that they have worked wonders for others and the cat has been around for a year or so longer.
You just have to decide if you want to take the chance and see if they will work for Allen. If you are willing to do the fluids (I never thought I would be able to do it, but I did it!) then give it a try. That's my opinion. But (as others said to me) only YOU will know when it is his time. I didn't really with Taz, but my vet carefully helped me understand. When the fluids weren't working there was nothing else we could do for him.
It is such a difficult thing to have to decide. But you just have to keep Allen in mind. Still to this day (over a year and a half after losing Taz) I have to remind myself that I did what was best for him.

((Hugs)) to you.