I read this book in elementary school but don't remember the title, author or anything aside from a few vivid points.
The story starts in the real world (I believe it's around WW2) where a boy somehow gains possession of drawings and plans for the atomic bomb. He knows these papers are dangerous so he hides them under his mattress and intends to destroy them,. but when he sneaks to his basement to burn them in the furnace he faints/falls ill and passes out, waking up in another world.
This world is similar but less technologically advanced. The world is somehow split / at war and he finds himself the centre of a prophecy foretelling a saviour with fair hair (which has never been seen before) would come.
I don't remember much else other than there's some mentor man that tries to help him but eventually the boy turns to the other side (the bad side I think) and discloses to them how to make the bomb.
He then realizes his mistake and somehow fixes it before waking up in his real world with a fever and realizing it was a hallucination. However when he goes to destroy the plans he find that they are nowhere to be found and remembers handing them to the man in the other world.
I think it has vague Christian themes to it if that helps.