Boyfriend with Depression

Kate
Im really worried about my boyfriend. He's showing some pretty strong signs of undiagnosed depression and he doesn't know what to do. I don't know what to do to best help him either though. 
I dealt with depression myself for about 4 years, even attempting suicide at one point, but I honestly don't know what to do to help my bf with this. 
Does anyone have any advice or suggestions on what to do? I straight up will not let him fall into this, but Idk what to do!