It was noticeable at the beginning of the season but as the season has progressed you can tell that he has gained even more weight. If he was 310 at the beginning of the season I would say he is at least 320 now. I think he could use about 5 more pounds of muscle but not more.
He's big. 315-320 is a good guess. I don't think he should try to gain any more weight. He's 23. Let his natural maturation curve kick in.
he doesn't need to gain much. before when he was pushed around i thought he needed much more, but now he holds his position against anyone and actually abuses people kind of. so i'd say 5 or 10 pounds more at most because he still gets pushed to the ground once a game, you don't see shaq punked like that often.
He falls because he loses his balance. 7'6" center of gravity issues. If anything, more upper body weight will make him fall even more often.