Jack is not so weak ____ you. A: like, B: as
So, I am wondering which one is correct in this context and why?
B is correct in my opinion; however, when you remove "so" as you point out, the answer can change. It really just depends upon the construction since "as" is a conjunction in this instance and "like" is a preposition. With the pronoun "you", a writer can get away with this since the subjective and oblique forms in the second person are always "you", but he can't get away with this when the pronoun is in third-person form for example:
Jack is not so weak as you (are).
Jack is not so weak as he (is).
Jack is not weak like you.
Jack is not weak like him.
I hope this might have helped you out. Take care and good luck!
As your 'intuition' tells you, the comparative construction here is not so ... as ...
This construction is fairly rare now—it has been largely replaced by not as ... as ...—but until a couple of generations ago many teachers and stylists insisted that so must be used with a negative.