My Frind Having Mentally Disorder, Please Help Him

Hello, my name is Phillip A. Im 16 years old and i have a problem with a friend. A couple of weeks ago during school, my friend found out his sister died. He started becoming quiet during our free time and started to listen to sad music and cryed uncontrollably. It was an extremely dramatic and sad moment for him. Later during lunch, i was told by a bunch of people that when i made everyone leave the room so he can be alone, he started cutting himself with sciccors that i later saw being througholy washed by my teacher. The next week he acted like nothing ever happened and even recently mentioned his sister's death in a weird way as if it was sort of like a positive answer to a question.i find it really weird that hes even close to sad about his tragic loss. Im not sure whats going on with him. Im kinda wondering now if he just staged the whole thing for attenshion or something is going on. i might think theres something wrong with him mentally. Hes one of those kids who never established a social relationship with a teacher, and our friends are "Incompetent" when it comes to anything serous like this and i feel like im the only person that seems like i would listen to him. i dont know how to approach this