If it's hair loss from DHT then he's right it very well could be gone because DHT weakens the hair folicle and makes it fall out. Since hair can only grow from folicle's, if they are not there, no hair can grow. However it very well could be something else like stress breaks like the hair being to dry and such (reason why it could grow back because the folicle is still there). Take preventative measures like the what has been recommended and hope for the best. I'm very prone to MPB and at 24 I'm as good and bald so I feel your pain on the hair situation. Good luck.