This happened to my family’s St. Bernard when he was about nine months old. He was put to sleep because he couldn’t stand. St. Bernards are known for this condition.
My parent’s dog had this as well, it was so awful. They tried everything but some things (supplements) only helped a little and only for a short time. Eventually they had to carry him so he could do his duty-—so sad. He lived 15 years but it was still sad when he had to be put down. I wonder of that Dr. Franks stuff they advertise on tv is any good.