The German War: A Nation Under Arms, 1939-1945 by Nicholas Stargardt.

What I love about this one, is that it's focus is on how the German people experienced the war they were fighting, and how this changes over the course of WWII.

I am by no means an expert on the topic, but I really liked how it felt a lot more vivid and real as Stargardt engages with a whole slew of first-hand testimonies from soldiers, schoolteachers, housewives, Nazis, Christians, Jews etc.

If he has a slightly cynical turn of mind, he might like Paul Fussell's several books on the subject. Fussell was a veteran of the European war, badly wounded, and liked to shock people about the realities of the whole thing. Wartime: Understanding and Behavior in the Second World War ; Doing Battle – The Making of a Skeptic ; The Boy's Crusade,

See also George MacDonald Fraser (he wrote the Flashman books), Quartered Safe Out Here about his time in Burma.

For a less pointed memoir by a decent man, Eugene Sledge, With the Old Breed about his time as a marine in the Pacific

He may well have read these already. A little more obscure would be the Italian veteran Eugenio Corti who wrote novelized versions of his war on teh Russian Front, as well as a nonfiction memoir, The Last Soldiers of the King: Wartime Italy, 1943-1945

Finally, and a little bit off track but really not much, is Amedeo: The True Story of an Italian's War in Abyssinia, the biography of Amedeo Guillet whose life really does sound too good to be true.

Moral Combat: Good and Evil in World War II is very good if what you're looking for is a general history of the war, it covers all the important stuff and personally I found it a very interesting book. The chapter on the Holocaust in particular is very well written.

Gerhard Weinberg’s ‘A World At Arms’ is probably the best single volume book on WW2. Its also huge so should keep him busy a while. It’s more focused on the bigger picture than the human aspect though.

Does he like the war aspect or the human aspect? I think Max Hastings' - Inferno: The World At War is a great place to start for a lot of people because it is VERY bottom-up. All of the chapters are chock-full of diaries, memoirs, interviews, and testimonials from regular people. It is SUCH a far cry from Shirer's Rise and Fall of the Third Reich.

It also covers most of the major fronts and even has a whole section about civilian life. If he has interest in a particular topic or front, it probably won't scratch that itch, but as a basic overview given texture by so many stories from regular people, I find it really compelling.