While he may be able to "handle" reading them I would avoid things like Harry Potter. As a teacher, I caution you to avoid pushing for "harder" books and ending up with them in over their heads. I cringe when I see little kids reading these books because while they can handle the reading level and get a little out of it, they can't truly appreciate the books. They miss so much and in many cases (not all, some kids love to reread) when they are old enough to appreciate them they won't read them because "I already read that." Plus, while the first book or two aren't TOO bad, as they go on they start killing off a lot of characters. Do you really want your 8 year old reading books about killing people?
My son loves Magic Tree House, Roscoe Riley, National Geographic for Kids, Ranger Rick, Geronimo Stilton. We also read a ton of nonfiction. We read a lot of easy nonfiction about animals and recently we found a great "A Visit To..." series about all the different countries.
When I taught third grade some popular series were Boxcar Children, the Ron Roy Series (A-Z mysteries, the Capital Mysteries), Encyclopedia Brown, Polk Street School Kids, anything by Matt Christopher. Two third grade books we read that the boys like: Stone Fix, The Stories Julian Tells. There is also a neat series about a mouse named Nicholas who is touring the states. This may tie in with some of what he is learning in school (may be more fourth grade). Magic School Bus (picture or chapter book series) is another favorite.
In general I have had good luck with silly books or nonfiction books with my son. Find out what books your son enjoys and go from there. Your library is an awesome resource. If you know of a book your son loves, you can tell the librarian and she can help you find more.
Also, like someone else said, don't get locked into chapter books. Magazines are awesome. Also, most picture books are written at a third grade level. It is nice that they can be done in one sitting though -- a healthy mix of longer and shorter books is good. Kids don't always need to be reading books that take them a long time and some kids shouldn't because they don't hold onto the info. Patricia Polacco is a great picture book author with third grade level books.
Another great resource I just learned about it www.lexile.com. If you know your child's reading level you can put that in, but if not you can pick a book that is perfect for him and type that in. It will then give you a list of other books at similar levels.
Here is a list of some books by level too: http://www.calcasieu.lib.la.us/SJWelshAR.pdf Again, if you know a good book for him you can use this to find similar level books.
Good luck -- I am so glad to hear you are keeping him reading!