How can u tell if your best friend is bipolar or some kind of problem,cuz i think mind does and i want 2 help.

my best friend has some problems i think. he has confided in me that something has been happening to him starting when he was 8 years old. ive tried to bring it up again but he always shuts down i think he wiishes he didnt tell me im a girl and hes a dude. his problems arent obovious, i dont even know if he has a probllem but his personality can change so fast... one minute he can be so happy and then on the brink of tears then so pissed off for no reason. i juss want to help