I was about the suggest "National Geographic for Kids" and "Highlights" but you got that covered. "NG for kids" is the best of science/nature type of magazines. It is hard to find one to replace it. Maybe you can try "Ranger Rick". "Highlights" is for slightly older kids than 6 years old. Your son will probably like it better. But "Ladybug" will do too. You can always upgrade it later.
My kids had these two magazines when they were young. Now they graduated to:
National Geographic: for science, multi-cultural
Economist: for current events, politics, international relation
Teen Ink: for kids writing
CICADA: literary magazine for teenagers
COSMOS girl: for make up, dating stuff. :-)
