I know that there are different flavors and such, but get whatever one is just plain. I don't think Tinker liked it, but when she was sick it brought her out of it in like 5 min. It's good stuff =) This could also be the reason why he is not eating. You know when you are really thirsty when you eat something salty? If you don't have something to drink you don't want to keep eating the salty food, just like their food is kind of salty. So maybe his not eating has to do with being dehydrated. Hope this works.
