I would never trust a vet to advise on a dogs food to be honest. Vets have virtually no training on a dogs diet unless they take that specific module of training, and that module is usually funded by Purina, one of the biggest dog food manufacturers.
Also Vets stock the top of the market brands where they make very good profits, so they will naturally push their own supplies onto you and give you all their spiel as to why you should feed it.
As long as their is no underlying medical problem with your dog giving him loose stools, just try giving him bones, that will firm up his stool very quickly.
Their is an enormous amount of great advice on the net about the best way to feed your dog, do some research and see.
Also, have a look at this.
http://barf-diet.co.uk/