i think the answer is
a. he did not perform enough trials.
but i want to be sure.

Sure.  From those choices, the only one that makes sense is that he
didn't perform enough trials.  Technically, you can't expect the experimental
probability to match the theoretical probability until you've rolled it an infinite
number of times.

I have a hunch that even for only 60 trials, such a great discrepancy between
theory and experiment is beginning to suggest that the cubie is loaded.  But
you really can't say.  You just have to keep trying and watch how the numbers
add up.