Quick update:
I took Hugo to the vet's this afternoon, and she couldn't figure out what the problem was. He is "very bright" according to her words, but his weight (it was 1.3kg before, now he's down to below 1kg) worried her a bit. He had been wormed at 5 weeks so was due another worming. She gave him his first dose, I'll have to do it again tomorrow and the day after. She said that worms usually make cats hungrier, so that might not be the cause of his loss of appetite.
She added that that reactions to injections are quite rare, but it could be a reason.
She said his tummy felt a bit thick in some areas, so if he isn't better in 2 days we'll xray him.
She also gave him an injection of steroids...
Hopefully he'll start eating properly again soon!
(can I just add how typical it is, that I had to go to the vet's today, when his insurance only kicks in tomorrow?
)