Usborne do lots of really good books about WWII in particular. There's a lift the flap WWII and loads of others. You can also get biographies of people like Winston Churchhill. Horrible Histories is good too. This is ds' favourite song
Ds was very into WWII at that age and there's absolutely loads out there.
Also some of the History Channel programmes are quite good for 5yos. They repeat everything in an excited breathy voice 
. Ds liked the Bismark one.
I was totally honest with him (and the girls when they asked) at that age, he knew about the Holocaust and Kindertransport, both of which have family significance (my great-aunt was saved by that).
I did disappoint ds though by refusing to build an air raid shelter in the garden, but he did have tape on his window to protect against breaking glass for a while, and he carried a box labelled "gas mask" until it got in the way. We did a week in the summer holidays trying to live on rations (the girls gave up quickly, and pooled their pocketmoney to go and buy sausage roles from the shop ) and found they loved rissoles, but even dh balked at the mock duck. Tbf that was revolting. Basically boil lentils in chicken stock. Mould into a duck shape. Serve. That was the recipe. I didn't think they'd eat it, so only made a duckling.
He still wants to fly a Spitfire and he's 10yo.
There's other books that he might find interesting if you read them with him. Ds liked the My Story set, but he also enjoyed "When Hitler Stole Pink Rabbit" at about that age. That's an autobiography of Judith Kerr who had to escape from Germany quickly as they were Jewish and her dad was a rather outspoken opponent of Hitler. That does make it sound rather scary, but it's written from the child's prospective and the parents kept them very much at that point thinking it was a rather fun game. There's very little really scary stuff-even my sensitive dd2 read it and loved it.
War horse is probably too old, by ds read it when he was a bit older. Knowing the background information does make it easier to understand.
A couple of other chapter books that give gentle info on WWII are by Noel Streatfield. The Children of Primrose Lane (catching a spy) and Party Frock (lots about rationing) just give a flavour of life during WWII.
You could also watch Goodnight Mr Tom, but check that first.