Help!

Okay so my boyfriend is bi. We've been together almost 5 months. He's just opened up to me telling me that he has a boyfriend who moved to America and got into a serious accident and has been in a coma for 5 months. I was supposed to fill the hole but then he seriously fell hard in love for me. I don't want to break up with him because I love him. How do ensure that I'll be the one he'll choose?