The cause was a sting by a cat
nail, that healed very quick, but the bacteria from the cat
saliva infected him; i understood that this thing is called cat scratch disease.
He is much better now, he responded great at the treatment with much powerful antibiotics. Also, after the ultrasound, the doctors could localize the pus and they could extract a large part of it.
I am very glad the problem was solved now, he has no fever, he started to eat and he seems to feel a lot better.