Cot dammit. He got sick again. This time they found hookworms eggs. So it is very likely that after all this time, the hookworms didn't get detected and it's the actual cause of his sickness. One of the vets has a similar case with a dog having fever and it's caused by hookworms. I worm him with Drontal Plus every 2 months usually. I'm worming him again right now with Cazitel for 3 days in a row and also Advocate spot on.