I know he isn't just a dog.. I know how good of a friend a dog or cat or any animal can be. 6 isn't really that old you know.. I mean it isn't young either but he still has quite a few years ahead of him! I'm sure he's alright and you shouldn't worry very much.. taking him to the vet would be a good idea though, even changing vets can be good. Simba sounds like such a nice companion. I hope for the best for both of you I know just know you'll both be fine.

-Fallon.