I am so sorry sweetie, this must be an awful time for you and I want you to know you are not alone. We are all here for you and we will do all we can to help and support you no matter what.
I wish I had some good advice for you, but I am afraid I don't. I know our kittens first started eating the cat litter but then stopped and they never got sick...but that doesn't really mean much does it? I really wish I could help more butI am clueless on this one.
I do hope the vet can help, and that you wont have to consider putting Oscar to sleep. I know this must be awful for you...and I just want to say I am here if you ever need someone to talk too.
Sending lots of good vibes and a friendly hug your way.