I am so very sorry. I too have a crf kitty, and I know it is a very difficult disease to deal with.
For me, though I realize Patrick's numbers must be very much lower, I use tricks such as bonito flakes sprinkled atop his food(the sold for pet varieties include Kitty Kaviar, Cosmic has a version, even cat in the hat came out with their own, fosters and smith came out with their own, etc.) which you can either get the "for pet" variety, or simply go to your nearest asian market and get them...they use them for making a broth and in other dishes.
Patrick also never turns down having wildside salmon cubes...whole or I crush them atop his wet food which he then eats some of while scarfing up the salmon. Rosie's Rosedust is another sure fire item he responds to, along with baby food (fed to him off a spoon, naturally).
If all hope of recovery is gone (wonderful sites if you've not yet perused them are: feline crf dot com
and feline crf dot org
I too would not worry much about what I was feeding. I might still consult the Katkarma food lists ( KatKarma canned foods list
and pick what he likes that is also lower in phos...they have an extensive list from prescription to premium to off the grocery store shelf listing of foods along with the important stats (protein content, phosphorous content)...it might give you some ideas.
Fwiw, crf kitties do go through crashes and abominal numbers...depending on what else is going on, sometimes they can be brought back for more quality time...certainly you and your vet are the ones in the best position to know if this is it, I do NOT mean to question that at all.
I would, if you are not, consider joining the support group using the link given in an earlier note.
I wish you all the best, and I am sorry for what I know is a very painful time,