I am sorry to hear about Picasso having diarrhea from this virus. I am a veterinarian and will do my best to help.
I have had luck treating cats (including my own cat) with diarrhea with Royal Canin Gastrointestinal diet. It comes in canned and dry, and in three levels: high energy for cats that are thin, moderate calorie for cats that are an ideal weight, and low fat if he is too heavy. You can get it from your veterinarian or at Chewy.com with a prescription from your vet.
I like to use probiotics, such as Fortiflora, Proviable or Culturelle. The first two are made just for cats and can be found online, in vet offices and in pet stores. Culturelle is made for people and can be found in drug stores.
A course of metronidazole (prescription from your vet) may be helpful. It reduces inflammation in the colon, and is an antibiotic that may help with overgrowth of "unfriendly" bacteria.
Adding fiber to his food, by adding cooked sweet potato or canned unsweetened pumpkin can help.
For the vomiting, I would try Cerenia (from your vet) or famotidine (Pepcid AC, 1/4th of a 10 mg tablet, twice daily).
Let me know what else I can answer or help with.