Remember I said you could not yell at or beat up the dog once he's there. Now, ten, twelve, twenty times in a row, you will do a comefore on lead. Be sure to do several of them in the spot where the original command was broken. After the first few, if you do them correctly, the dog won't want to get away from you. You want to make your point VERY clear.
Prior training is essential.
He must also be exercised and trained regularly. Your dog can too. Treat this command as a matter of life and death, and train it that way, because it is.