It's been seven difficult days, but he seems to be recovering now.
It all started last week when I found him curled up on a chair, breathing with some difficulty.
It turns out he had a pleural effusion, the cause is unknown, but the origin is bacterial.
The vets had to sedate him and put a drain on him to remove the liquids from his pleura, and he was only allowed to be back home yesterday after they made sure there is no liquid anymore.
He is taking two different types of antibiotics and needs to be watched closely in case the issue comes back.
He is not entirely out of the woods yet, but at least there is hope he will heal completely.