Did you ask your cousin if something had happened the second week while in her custody? That sounded like he may have gotten scared or stressed prior and now not acting himself. Did you also check the water parameters before you made changes to his water just to rule out any chemical imbalance?
I know it can feel like betrayal when they up and ignore and behave the complete opposite, but may be give him sometimes and continue to observe carefully for other signs. They have short memory, so they can't stay mad at you for a long time (at least that's what I like to think).
Good luck! Hope it's nothing serious, just acting out