[Cap.Jang Cat] 28.SOS, My kitty is sick. I don't know what happened to him (内の猫がいたいです)

He was my kid and kitty born in my room last year. He got sick suddenly. He's my family I want to help him but the vet doesn't know the what the sickness is. I'm sad. just sad. Now it's 3 days.
My big brother J.A. gave me some ways and how to deal with it. But if anybody knows something, please let me know. Please.
male 1year 3 months

he got fixed last week

I really didn't want to make him castrated

but after 1 year since he was born

he was so coward and never tried to leave home while neighbors' cat were leaving their home.

After the operation 2 days passed

he started to vomit

and stopped eating.

he vomits 10 times a day.

next day I brought him to hospital

his temperature was normal.

The vet couldn't find his sickness

now 3 days

what should I do?

He was ok last month.