I would let him believe for as long as it's real to him. The belief in magic - fairies, gnomes, Father Christmas, etc. - is such an essential part of childhood that it's something we cherish long after we stop believing. And while it's important to teach children responsibility and a good work ethic, it's also important to foster their ability to dream, pretend, and imagine.
That 'magic' of childhood is something that we go back to continually in our adult lives. I believed in Santa Claus for real until around age 11/12, but then I continued to allow myself to believe until I was 15 (at which point, I just couldn't convince myself anymore, no matter how much I wanted to) because I enjoyed the magic so much. Your child is still pretty little, so I don't think there's anything to worry about.
My brother had to be told because he continued to avidly believe in Santa Claus and the Easter Bunny until he was about 13, but 9 years is still completely appropriate to believe. And if your child ever asks you if Santa's real, I used to always say, "Santa Claus is real to those who believe in him.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.