That picture of him is so adorable! When we first adopted our Darko, his name was Jerry, and they look so similar
. Anyway, have you been able to determine IF it's possible he ingested some sort of toxin around your house...perhaps a plant of some sort? Maybe someone even gave you some fresh flowers? Does he go outside at all? There are so many things that can cause this, and Easter Lilies are one, but I think it's a little early for those, although I guess it depends on where you live.
My thoughts and prayers are with you and Jerry.
My thoughts and prayers are with you and Jerry.