Hi, Jay,
How big were his parents?
If I were worried about my dog's growth, the first thing I'd do is take him to his vet. You seem to be concerned only about his height, so I assume he is not overly thin.
I don't know exactly when a dog stops growing in height, but 29 inches is pretty big for a St. Bernard -- take a look at
American Kennel Club - Saint Bernard for the AKC breed standard.
kjd