Was he always like that? Has it only recently become so?
If he was like that all the time, how do you handle it? It's like living with a bomb, you don't know what to expect from it the next minute.
Personally, I try to avoid such emotional people because my nervous system can't stand them. I am a calm person and I am comfortable when there are also calm people around me.
If he was not like that before, then maybe he has some problems?