Thank you all for your thoughts and prayers.
Right now the news are a little bit better. Filou was fine in terms of (bad) behavior on the weekend. He also started to eat - about 20 portions per day, some of them as few as three kibbles but also including one egg yolk per day and some minced beef. So he has gained an ounce or so which makes me happy as at least the weight has stopped to go down.
The vet considers again homeopathic meds. They are a little bit easier to administer as they are very small pills or drops so I think we'll give it a try. He had them at several times during his disease but we'll stop it if he gets annoyed with it and stops eating.