This was going to be my suggestion.
Actually, if you want to go a bit older school (ie, World War I) then you really can’t go past the Biggles books by Captain W.E. Johns. They’re fictional but the setting and equipment etc is spot on (at least in the WWI and WWII-set books), along with the general zeitgeist of being a pilot in the Royal Flying Corps in WWI. Some of the later books were set during WWII and had Biggles & Co. fighting the Nazis, and there were others set in the 1950s and 1960s in which Biggles and his chums are “Air Detectives” who fight crime using aircraft.
I grew up reading them and they’re absolutely perfect for a 12 year old (They were written for what is now called the “Young Adult” market), with the benefit of being full of adventure and clear good guys and bad guys. And aeroplanes.
For a “serious” WWII book, I can’t recommend the Reader’s Digest book The World At Arms highly enough- full of colour illustrations and maps, easy to follow text that still explains what was going on quite thoroughly to the lay reader, and generally one of the best overviews of WWII I’ve read.
Also, don’t underestimate the value of computer games for education- things like the Total War and Civilisation series are chock-full of factual information and it’s surprising how much you can learn from playing them. That and it’s worth bearing in mind that information on a computer screen is presented in words as well as images, so he’s still “reading” when he’s playing computer games- just not from a book.