Infinity/eternity is frightening to anyone who really thinks about it. It can be especially troubling to the recently ex-religious, because they've lost their particular assurances of perfect eternal happiness. Compared to that superficial idea, the prospect of simple unending persistence is much harder to shake, because we all have a tough time imagining ourselves not existing.
Your boyfriend's on the right track intellectually, because he accepts that there probably won't be a him after he dies. We just need to address that small probability that there will, and settle his emotions a bit.
Right then, what if he IS wrong? Anything is possible. The odds of any particular afterlife story being the real deal, including whichever version he may have grown up with, is almost zero. That's because there are an infinite number of possible afterlifes: the thousands imagined by humans in the last few thousand years, plus the countless ones we haven't thought of yet.
You haven't said what he specifically fears about living forever. Maybe he hasn't really told you. He might fear torture in hell, or a post-life sentence as a ghost watching everything he knows crumble away, or imprisonment in his own disembodied mind deprived of all senses, going mad in darkness and silence. That last one sure gives me the willies.
If anything is possible, all of the above are. Is any one of them at all likely? No, because they're swimming in an endless sea of alternatives. Put simply, whatever he's afraid his ultimate fate will be if he doesn't end at death, he's almost certainly wrong.
The only death scenario for which there is any solid physical support is oblivion. The mind, the ego, the identity, the memory and everything else that comprises "you" is contained and operated in a physical, bio-electrical brain. After death, that brain first shuts down and then disintegrates. It's a materialist view, I know, but arguments against it centre on things the physical brain supposedly can't do, and without exhaustive analysis are arguments from ignorance.
So take your boyfriend's most troubling scenario and work out the chances: the probability of an identity outliving the human brain outside of all detection, multiplied by the probability of that scenario being the one real one out of infinity. It's like the chances of finding a particular blade of grass if you don't know which country it's in.
That's the thrust of my recommended reassurance. To sum up, if there's an afterlife, you can be confident that it's different from and therefore less scary than his idea of it.
First of all, consider the possibility that your creationist, let's call him Bob, didn't know the Truth himself beyond a vague concept. Either that, or he realised his idea of the Truth wouldn't make sense to someone who wasn't conditioned to accept it unquestioningly, or simply wouldn't convince a real live atheist. Maybe Bob lost his nerve and accepted that he was no apologist.
I'll take a small leap and assume that Bob the young-earth creationist is a Christian. The Christian Truth is just the business end of Christian belief, the critical bits you have to accept to be a functioning Christian:
- God made the world and everything in it, and rules over it.
- Humans are all tainted with Original Sin, plus all of their own sins.
- Jesus Christ was the son of God and he died to save us from all those sins. God then resurrected him.
- To get your share of this salvation, you must 1. accept Jesus as your personal Lord and saviour and 2. confess all your sins and be absolved. You must keep these two things up for the rest of your life.
- Do 1 and 2 until you die and you will go to Heaven. Fail to do either, and you will go to Hell.
- Aside from all this, both God and Jesus love you.
Young-earth creationism is the direct result of belief in biblical inerrancy, which means that every word of the Bible is the God-given truth (with a little t). Geology, biology, astrophysics and other branches of science very plainly contradict the Book of Genesis, for one, which throws into question not only Creation but the story of Original Sin. Christianity needs that Sin as a hook; it gives everyone something he/she needs absolved. It's like a pest control ad which says everyone might have cockroaches in their walls.
So believers take on science indirectly, not usually trying to change the established theories but campaigning to teach the alternate Bible-friendly versions to people young, devout or gullible enough to believe them. That's where Bob is coming from.
Bob actually thinks that you only claim to be an atheist. The only reason you might be afraid of Bob's Truth is that you think it really is true but you're in denial. In other words, you're a closet Christian trying to avoid responsibility for your sins.
In your next round with Bob, in order to advance his understanding your task is simply to convince him that you are an honest-to-Bob atheist. To do this, you might have to convince him first that there's even such a thing as an atheist. If he acknowledges your genuine atheism it should become obvious to him why you're not afraid of what, from your perspective, is an Untruth. Then he's got to convince you it's the Truth from scratch, and you've both got a much more reasonable conversation.
How do you demonstrate your atheism, you ask? Ironically, through your knowledge of Bob's Truth. A common, almost unconscious assumption among religious people (and many others) is that to be exposed to their beliefs is to adopt them, almost universally. You preach, you convert. The fact that you have a clear idea of what Bob believes and yet reject it might be a bit of a shock to Bob on some level.
Best of luck, and comment here later to let us know how you went. May Bob be with you.