I was right, poor Milo has intestinal parasites. Also ear mites. So he's basically quarantined from Ozzy. We have to give him ear drops every day and dewormer for 6 days (2 more days left out of 6).
But the poor baby has diarrhea :/ I don't know if it's because of the dewormer or because we switched him from wet food to dry food. The vet is also on vacation until the 11th.
Otherwise Milo seems to be doing ok, he plays a lot, likes to jump while chasing toys. He purrs so much! And meows a lot too! We're having conversations with no problems. He eats well, drinks a lot of water too which is good considering dehydration is a risk of diarrhea.
I hope he'll get better soon :( he's such a sweet baby! Compared to the smol devil Ozzy is, because Ozzy bites everything in his path XD