Book Suggestions (3rd grade level) History and/or Adventure,

My son who just began his 6th grade year, reads on about a 3rd grade level.  Some read alouds that he has particularly enjoyed are The Green Ember, My Side of the Mountain, The Bible Smuggler, The Sign of the Beaver, A Lion to Guard Us, just to name a few.  He also loves history and/or adventure.  As our books for the upcoming school year came in over the early summer, he read the back of the book, Snow Treasure, and said that sounded like his kind of book.  He has been discouraged because many of the books that he is interested in are too difficult and he struggles.  I want to challenge but not frustrate.  So, we are trying to find some books that interest him.  I will expand his horizons on books in the future, but right now I really want to focus on things that interest him.  I also think some biographies might be interesting to him if they tie into  an exciting time in history.  He particularly loves WWII.  Any book  recommendations that would fit his interests?  I am trying to help him become a stronger reader and build a stronger foundation while building his confidence this upcoming year!  Thanks for taking the time in advance!