Hi Chevymaro92 - welcome to the forum!
Those symptoms tell me that he really needs to be seen by a competent Vet as soon as you can get him in.....
I can't believe he was hospitalized for a week, then discharged still unwell.
Is there a cat-only Vet clinic close by?
If not, you could start to look right here: Find a Feline Practitioner
I'm also thinking that he's experiencing pain as well - the unusual vocalization might well be a symptom.
What were his original symptoms (before the hospital stay)? Was he urinating outside the box and vocal then too?