Thank you all very much for your compassion and advice. It means so much to me- in an emotional as well as in a practical way.

What I'm planning right now is the following:

Sub-Q fluids: It would be very good if we could learn to administer them. Since Tuesday F. is very well- has even slept on top of the cat tree, has chased his big brother and did not drink as much.
What I'm worried about is that he is a wiggle worm. He never ever sits still for human attention, even when he is in a cuddle mood he does it by walking your shoulders, always in control and always aloof when you try to hold him. You can always approach him with your head and never with your hand .
I will talk to my vet about it.

Other vet:
Some of you will remember that one of my co-workers rescued a Spanish dog during his holidays. Chulo has a Ehrlichiosis and Leishmaniosis (some bacteria very common in Southern European stray dogs who on the long term can lead to death). He had several rounds of antibiotics but his levels got worse all the time. Then he went to see a homeopathic vet and from then Chulo got better- you could see immediately how much more lively he was and his blood leverls improved as well.
And my other co-worker Gudrun's Teddy (now RB) had an eczema years ago which also disappeared after the homeopathic vet treated him.

I now have made an appointment (in two weeks time) with the same vet who treated Teddy. I called him yesterday and talked to him for 15 minutes- a good sign for me. He is a real vet with an additional homeop. grade. He asked lots of question and said he had the impression that Filou is in an early stage (we all think that) and that homeopathy should be able to slow the development down and to add life quality.

My normal vet is a cat specialist. I trust her fully but of course I will inform her about the appointment. The homeopathic vet asked me to do that but I would have done it anyway. He wants to co-operate.

So we'll do that and I'll keep you informed.

And thanks for everything. I am so happy I joined this board