It will most definately make it harder for you, if you do lose him, to remember that you had sex with him (especially if you are a virgin).
I know you feel pressured, but I still don't think you should give into sex if you are not ready. I know this sounds mean to tell him no when the doctors say he may die, but it could affect you psychologically if you have sex due to pressure. Also, if you read my paragraph below, you may also have additional reasons to abstain sexually with him for now.
Just keep positive. He'll be alright. He won't die. Tell him this. Talk to him about the future together, and how he will be perfectly fine, and all the years you will have together. And believe it. Don't just SAY it, BELIEVE it. Having you believe in him, and abstain from sexual activity because you know there will be time to be intimate later (and when you feel you're ready) will make him feel better about his prospective future, and give him strength to fight the cancer, and give him more reason to believe that he will be alright. You have no idea how much simply BELIEVING you are strong enough to fight your health problem and/or having someone else believe you will survive will actually HELP you to survive. If you believe you will survive, your immune system automatically gets a kick start and gives you additional strength to help the health problem. I've seen 2 miracles in my life where this has happened. It was nothing less than utterly amazing. Both people were diagnosed by doctors as not having a chance in the world to live, and their friends and family were both told to prepare for their funeral. But both of these people (one was named Janie, and the other one was Serge. Janie was a 9 year old girl with cancer, and Serge was a 64 year old man) insisted over and over that they were going to be alright, that the doctor's warnings about the likeliness of death and the tests were wrong, and Janie and Serge talked about the future as if they were truly going to make it. And, amazing, though only few people believed both Janie and Serge, they miraculously pulled through. But, damn, they both believed with all their heart that they would survive. Sincerely, it was nothing short of amazing.
Believe your bf will be alright, and get him to believe it. I am not a believer in God, and I admit to being an atheist. And while I do not attribute Janie & Serge's survival to any sort of supernatural being or diety, I have seen the forces of just BELIEVING at work.
Please, please, don't lose hope in your man. Give him reason to live. Follow my advice.
There will be a tommrow. There will still be a tommorow even months and years from now, and both you AND him will be there to see it. All you have to do is believe.