Should I marry a dying man?

I've known this guy for 10 years and we've always been strictly platonic. However, 3 months ago he told me loves me and that he has the whole time. I knew I felt something for him too, I was just cautious to call it "love". But the problem is he was diagnosed with leukemia 3 months ago and has very little time left. Last night he asked me to marry him and all I could do was cry. He told me he understood if I declined because he said "I know it's incredibly selfish of me to ask this, but it's all I ever wanted. Just you." I know it will hurt tremendously when he's gone but I'm pretty sure I can do this. I think I can take this huge risk and that I love him enough to sacrifice my "safety" and happiness for his. What would you do? Please help, I'm very young...not even 20 yet and I've never had to deal with something like this...