Your question is good, but I can be very clear when I say that a person cannot die if he or she doesn't have sex. There are many myths and misconceptions about sex and it is always good to make sure you have the real facts to understand. You should not pay attention to what would happens to a boy if he doesn’t get what he wants, but to your own feelings and decision. You matter before anything else. Think of your decision and how you would prevent any consequences that may be a result of it.
For instance if you have accepted his proposal, and there is a risk to get pregnant, how would you prevent that? You can consider using condoms, which will act as a barrier to stop the boy's sperm from entering your body. Condoms will also act as a way to prevent the risk of attracting HIV or other STDs. Or you may have decided to abstain, in which case you should feel confident to say no to him without fear of any negative consequence happening to either him or yourself.