we love muppets in space SOO much! It's one of those ones that the kids can watch it day in and day out and mom and dad don't lose their minds hearing it over and over...it's clever and intelligent and so on.

there is a veggietale that goes in space...it has the good samaratan part at the beginning, but then the second part is the "USS Applepies" (star trek spoof) I think it's called "God wants me to love them" or something like that.
Honestly, we felt fine about our DS seeing the original Star Wars movie at that age. We sat with him the first time to make sure he wasnt freaked about darth vader etc, but he was fine. He also saw Phantom Menace around that age...we waited a bit on some of the otehrs in teh series (he's 8, and will not see Revenge of the Sith for a loooong time yet) but he's seen all the others now.
Of course there are all the star trek ones...don't know how you feel about them rating wise...we haven't let our DS see them yet, although DH loves them and I know the time is nearing.

Again, rating in question...but men in black is full of aliens.
War of the Worlds is an old classic--might be too slow for him, but it's not as scary as some of the newer alien ones...