Oh no, I'm so sorry to hear this. I will be praying for your sweet Sebastian.
I can't vouch for this product yet, as I'm still waiting for it to arrive...but I've read a lot of good reviews online for Rosie's Rosedust. Maybe you could consider this to jumpstart his appetite.
It's so hard when they don't eat..I'm going through it now with my cat, Nipo. You and your furry family will definitely be in my thoughts!! ((((HUGS))))