keep an eye on your dog mate,the weight loss could be do to something else intestine infection,it might not be but if this persists take the dog to a vet,my pup was the same would eat but wouldnt put weight on and had the propper runs,took im to the vets gave im a antibiotic injection cost £30 hes now sound as a pound,its not always to do with food change,but stick to one diet dont keep changing,it doesnt help is digestive system best of luck with your dog hope all is ok john.