Puppy with 3 legs?

We have a German Shepherd puppy who is 6 weeks old who is missing his right front paw. (Vet said when he was born he had the umbilical cord wrapped around his arm and when mom tried to take it off she took his paw with it.) His leg ends about midway between where the 'elbow' joint is and where the paw would be. Vet said that if he tries to walk on his 'stub' we will have to have it amputated at the shoulder. Well like I said he is 6 weeks old now and when he walks he uses (or tries to use) his 'stub'. Is this because he is so small and doesnt have the strength to walk using just one front leg...or is this just a habbit he has that wont go away and we will have to end up amputating whats left of his arm? Opinions please!