We use the McGraw hill big workbooks for language arts. I like it because they are cheap ( >$10 at Sams club), cover a lot of different things, and are easily done orally

. Nice to have around not so expensive that you are a slave to it. They roll along pretty easily too. My dd never stumbles over anything evenifit is totally new. They just present it in a very understandable way and it is so smooth. Like I said we do most ofitorally to so we can whip through 4 or 5 pages in about 10 min. Very low stress.
As for themes, Go crazy. If there is a theme you think he might like do a search online for that topic. It would be my guess that something from
www.Easyfunschool.com will pop up and they always have cute ideas, My 7 year old and 4 year old are about to embark on a journey into the world of pirates. and then I found a chocolate unit study which is just too fun not to do.
www.About.com also will give you a wealth of information on any subject.
Somethings that we like to keep on hand to make any book into a unit study:
poster board and end roll paper (you never know when the mood may strike)
globe/world map
empty notebooks
and soon we will have the book "a time line for all times" which is a time line in journal format. It is hard to explain but there is enough room for her to enter every event we will talk about now until the time it decomposes. It starts baout before Christ and goes up until the year 2010 at least. Has clip art for famous people for you to insert and some specific events have expanded chapters. there are also maps and diagrams. it is awsome.
Really any book he is reading can be made into a unit study. What is the setting (geography) when did it hapen? what else was happening (history) math is sometimes an easy tie in but sometimes not, write a paragraph about this chapter (language arts), why did this happen, how did this work? (science) Do you want to try this game they were playing? (PE) The possibilities are endless. run with it. the library is also a valuable source.
www.booksbloom.com is a great resource for books and Jan Bloom can tel you exactly which books will hold a boys intrest and how to get the most from those books. And they have a lot of good books.