I think they both have their moments, but if I were going to have to pick one, I would say that the boy is the one who is in control of this relationship. I think that he knows a bit better how to manipulate his mom.
We see, for example, that she will get him goggles just because he wants them. We also see, more importantly, that he is willing to go off on his own without asking permission. I think we can also see it at the very end of the story where the mom thinks she understands what is going on and is ready to argue. But the boy is really in control and he has already decided that he's done swimming.