My dog is 16 and today he started throwing up white flem and his eyes rolled back, and he cant stand on back l?

Take him to the vet IMMEDIATELY. Your dog has lived 4 years past his life span. Most dogs live between 9-12 years. Smaller dogs live a little longer. Large breeds like St Bernards, Danes and Mastiffs only live 6-7 years. Get your dog to a vet. Sounds like he had a stroke.