My boyfriend has cancer and is dying, what should i do?

My boyfriend of almost a year has been battling leukemia for a while now he is only 16 but the doctors caught it to late and he is now dying. I love my boyfriend so much but I just don't think i can sit back and watch him die and I feel utterly helpless. Although I love him I'm not sure if I should leave him or stay with him, please help?!