If he loves math try the Singapore Math books. They come with a Textbook, Workbook and Practice Books. You can download a pre test for free to see where DS is before you buy and then just take him through the curriculum at home on your own.

http://www.singaporemath.com

Do you have any nice museums in the area? We have found the children's programs to be amazing. Also if he loves reading you could see if there is any local theatre then get the book, see the play(or movie or dvd) and discuss the difference. How about the zoo? You could get books on animals out of the library and then go to the zoo and try to identify groups etc...