We don't have cable, so there are many pop culture things my son doesn't know about, although he's only 3 so the social aspect of fitting in isn't as much of an issue. My son actually first learned about Mickey at pre-school. Still, the only 2 Disney movies he's seen are Cars and Nemo, which we own and I don't let him watch them very frequently. I'm going to take him to Toy Story 3, mainly because he loves watching movies and I think he'll enjoy the in-theatre experience. At a family event recently, my mother-in-law bought a bottle of Sponge Bob soap for the kids - all of his cousins were so excited, but my son was clueless. More importantly, it didn't bother him. You could try getting DVDs from the library to see what other characters or shows he'd like.
Other than the usual boy stuff, my son is also crazy about space, so we take him to the Museum of Science and Industry, and the Planetarium has a new (permanent) exhibit called Planet Explorers which is geared for younger kids (3-8). MSI has an original Mercury or Gemini Capsule, as well as one of the lunar lander test modules. We have just about every toddler book about space, and they're great because they're factual: learning the planets, books about the first moon landing, learning to be an astronaut. If he's obsessed with numbers, On the Launch Pad is a space-themed counting book, although it's only up to the number 12, so it might be too easy for your son.
Anyway, I don't think you should worry about it too much right now. He'll get exposed to it and learn from kids at school and will probably ask you about it at that time.