My Jack Russell Terrier is sick

We have two JRT's, Reggie & Ruger. They are 11 years old and brothers from the same litter. We got them when they were 7 weeks old and they have always been healthy. Our Ruger suddenly stopped eating and now has a fever. I've had him to the vet twice and have given him meds but nothing helps. The only thing he will eat now are his treats, biscuits and soft chewy treats.

This started after Purina changed the Proplan dry food to the shredded blend. Although the dogs loved it, they started having issues with pooping alot more, losing fur, etc...since then we have tried several different foods and that is when Ruger stopped eating.

Everyone says he will eat when he gets hungry enough but that is not happening. I have tried giving dry, canned, ground meat, cheese and he will maybe eat a very little bit or just turn away. He is still drinking some water and will sometimes take some chicken broth. He has lost a lot of weight.

Of course being a jack, he still runs around the yard like a nut chasing deer and critters but don't know how he has the strength to do it. I am at my wits end and very worried for him....

Well, I just needed to get this off my chest...sometimes other people don't understand that my jacks are my kids.