Refused to take back shoes

the manager refused to take the shoes back because the shoes is no broken or torn. she can see the problem but insist that she will only take it back when it broken or torn. She doesn't know about the factory or manufacturing fault. The left and the right side are not the same, the other one is smaller and it presses and hurt my son foot. who can I allow my son to continue wearing it if it will cause blisters. My son only wear the shoe once. And he prefer to wear his old shoes than the new pair.