I am really glad your vet at least mentioned pancreatitis. I have a kitty with this and it is NOT fun. It has been on and off for the last two years, but my previous vet didn't recognize it. Turns out my kitty also has a large gallstone, very uncommon in cats.
I have to take him tuesday to a specialist and a surgeon to determine if his gallbladder needs to be taken out, or if something else is causing the repeated attacks.
The only signs are vomitting, then off his food. I don't free feed so I notice it right away. Then we start the cyproheptatine, prednolisone and buphrenorphine. At first it would last just a couple of days, but now it has progressed to weeks. This current attack is going on day 10. I have a post here when he first got diagnosed, but I need to update it. I wanted to log about it for anyone in the future that may search the topic, as I can't find much about gallstones and pancreatitis in cats.
Hope your guy perks up really quickly and that this doesn't occur again!