But - when we first learned that he could walk, when he terrorized his father, he did seem a bit unsteady and awkward. Then, with the reading of the will, when he "almost" walked and Gerard commented on it, his feet were crooked and it was very uncertain. At that point we assume that he's managed to get around on his feet, but probably not too well.
Now - he goes racing around the house, running down stairs, etc. as though there never had been an accident.