How to help your partner with depression ?

Me and my boyfriend have been together for a few days. I love him already he's my best friend I known him even before we got together. He's older than me he's 24 I'm almost 18. He's really nice. He's also really sensitive he also suffers from bad depression. He gets upset and cries at every little thing. He has fits and acts like a little kid. He even tried to cut himself at one point he nearly died. He's gotten past that but he still cries once in a while at everything. Me and him and our friends have a band yesterday we were practicing he got pissed off because our manager touched his drums he doesn't allow anyone touching his drums he got mad he yelled at our manager he would have gotten into a fight with her if me and her brother hadnt stopped him and kicked them. Then she told him to go sit till he calmed down. He does little things like this all the time. He has mad fits at everything and cries at everything too. He never had a really bad childhood his parents were always around still are he didn't have any brothers or sisters he was an only kid. He did say he had the perfect life growing up but now he's really depressed and upset about everything. Even has days where he breaks down crying for no reason. How can I help him get through this ?