When Colin throws a temper tantrum after being exposed to an open window, the camera returns to focus on Colin for reaction shots several times during the argument. In some shots, he's dry and his face is pale; in others, he's flushed and sweaty.
In the final scene, while Mary, Dickon and Colin are playing Blind Man's Bluff and Colin discovers his father in the garden with them, the blind fold (which Colin has pulled up on his head like a headband) changes disappears when the camera is looking over Colin's shoulder.
If Colin had really spent his entire life in his bed and had never walked, his muscles would have been severely atrophied, and it would require years of therapy to rebuild them. He definitely would not have been able to walk as well as he does at the end of the movie if he had just started walking, at most, two months before.