the whole reason Sherman sought an escape from society as a whole was that he wanted a leisurely circumnavigation of the globe via hot air balloon because he was burnt out from years of student antics. and as the professor sets off he is no longer constrained by society.
and the antagonist of twenty-one balloons is not a character but an idea. Sherman battles against the blind acceptance of how things are. he values innovation, so the protagonist would be society's stagnate nature.
I hope my insight has proven helpful, have a good day!