My St. bernard started behaving the same way just recently. We tried everything for over a week & then called the breeder for help. he said yor dog might be having fever, n that was it. Actually, he was having a sore throat because of which he had mild fever and had lost his apetite.
My Vet advised me to give him Aptiquik (medicine to increase apetite in pups) twice a day (5ml). Along with some antiboitics for sore throat n fever.
He has started eating again. I give him powdered Royal Canine mixed in lukewarm milk(diluted with water) + half a roti +egg yellow. Sometimes I mix it with atta bread ... Keep changing the flavour a bit, n your dog would eat it.
Also, stop giving him puppy buiscuits, sticks, treats etc for a while as they kill apetite. (I stopped them)