Seeing my 5 year old son was interested in weapons, fighting and monsters, I introduced him to Greek mythology-Trojan War. He was delighted with this and was soon on to series of books called Beast Quest. I personally think they are horribly repetitive after you have read the first 20 or so (we read the first 30 books together), but they got him in to reading in a big way.
Now in Year 7, my son has read hundreds of books on battles, wars, weapons, monarchs and history in general. Watched many related movies, been to a large number of museums and historic buildings (this does make holiday destinations a lot easier) and watched too many topical You Tube videos, many of which are absolutely fantastic.
For quite some time he was using an app where Hannibal was fighting against the Romans and he now seems to have a near Encyclopædia knowledge about how all the battles of the Punic wars were fought.
As I write, he is swotting up on some old papers for a history Competition that he will be entering this Friday: www.townsend-warner-history.co.uk/Townsend-Warner/Home.html
I would suggest Horrible Histories is probably a bit mature for him as my son still reads them now if we come across a new one.
There will be a number of DK books that will be suitable, as will many many you tube videos.
I don’t regret nurturing his interest in these topics at all.