You might disagree with his premise, but that is the premise of omniscience as a characteristic of the Judeo-Christian Triune Godhead. Arguing about the definition of the word doesn't change the validity of the position. So, our God, being omniscient and eternal exists outside of time and space.
Your argument would be much better constructed based on the premise of a truly Sovereign entity in correlation with a being (obviously under the entity's sovereignty) who at the same time has the coexisting trait of "free will".
Omniscience in no way forces one to act. Omniscience merely knows the action which will be made. Being able to see the future (if you will) does not mean that you in any way control and/or manipulate that future. This is why your argument would be much more defensible in a premise of Sovereignty and Free Will.
1) However, consider this, is it not possible for my free will to simply be on a perceptional basis?
Of the two entities, I am the lesser, thus my characteristics are those that are finite and mutable, and the greater entity is that whose traits are infinite and immutable. The greater entity obviously has a perceptional capacity with much greater vastness than do I, as a matter of fact infinitely so.
The question to then pose concerning my inferiority in characteristics is my perception of my alleged free will.
From my viewpoint, I can act entirely of my own discernment and choosing. My finiteness and mere knowledgeable existence does not allow me to see any of the supposed sovereignty superimposed by the greater entity.
2) The other perceptional basis of free will would be concerning limitation in choice, but still allowance for freedom of choice.
I will use your own analogical premise to prove this point.
Box A and Box B sit before me. I choose Box A. Who limited my choices to only Box A and Box B? This has left me with a mere four choices; choose A, B, both or neither. There is nothing else I can do, thus obviously some sovereign entity has already superimposed His sovereignty over my free will in some respect. This perfectly demonstrates that a greater entity must exist.
If I had free will in an exhaustive essence I should have no limit to the choices I could make, that would be true free will as you labeled it.
Example:
Another simple analogy to portray this is a teacher giving a multiple choice test. You have free will to pick any answer for any question you want, however, the teacher supersedes your free will by limiting your answers to those already provided by them.
Does this mean you have no power of choice, no implementation of will? Absolutely not, it means your implementation of free will is not exhaustive.
Your argument can be entirely valid to show two things:
1) There is a sovereign entity in existence.
2) My free will is not as exhaustive as I may want to believe, however, I have still been allowed the implement it.
Thus, without you personally defining true free will, and thus my own interpretation of your intent, I would say you are right.
True Free Will does not exist within our finiteness in correlation to the Sovereign Entity which supersedes our will. However, true Sovereignty must exist, as even your own analogy indicates.