Aww thats too bad
I thought pet stores everywhere would carry those.
I don't know if Amazon delivers internationally to London, England or not? But they're sold on there, and can be over nighted if they do deliver there:
The salmon flavor is the most tempting, at least for my cat. Even when she's not hungry, that flavor gets her to gobble it up each time. I try to avoid the salmon, since fish-based foods are bad for them, but it doesn't hurt once in awhile just to get the pill down them. She does eat the chicken flavor too though, but not as thrilled about that flavor.
Maybe a new smelling treat would entice him. Good luck, I really hope you can get him to start eating again. Poor guy, hate to see them sick.
