I'm so sorry to hear that Willy has tummy problems. I know that keeping him away from water when you have other pets is hard to do. Maybe he needs to be put in room by himself for a while if he'll tolerate it. I've had this happen with my Storm before and every time he'd drink he'd also throw back up so I had to take the water away from my bedroom area and put him back in my bedroom. After his tummy has calmed down a while, you may want to try giving him 1/2 of a pepcid pill regular strength. Storm takes one every night to help with his tummy issues and I've also used it in the past with my other cats when they've had tummy issues. I hope he starts feeling better soon.