The vet called this morning: the big bloodwork showed that it is not exactly anemia- his levels are at the lower end of normal.
She will think about medication. (Hopefully no pills because this frail 6 lb bundle of energy is ab-so-lute-ly unpillable - you are always afraid you could hurt him and he takes advantage of that.)
Now it's all about gaining weight- Filou is allowed to have as much cream and even carbs (which usually are too high in most industry brands of cat food) as he wants. Only the protein should be controlled. Just opposite to what I should eat

After all a much better outcome than I expected.

Thanks for your help