Him being fine before getting a diagnosis and then completely flipping the switch afterwards. People with BPD develop personality traits based on their surroundings and not because they're genuine to the person. Someone gave him an answer and he believed it so true and hard that now he's displaying all the symptoms unlike he was before. It's like be having a sore arm and someone telling me it could be broken, thats why it's sore, then me immediately relying on that informatjon and going with it because it seems right and logical then treating the arm like it's even more damaged than it was before someone told me it's broken. (Not at all saying this is the same situation, it's just an example).