...is to be friends with someone from that culture!
Hopefully it's easy to find someone of another culture in your community. Maybe your child has already developed a friendship at his school who has a different cultural background than he does. The best way to learn that culture is to invite them over for dinner, or for a playdate, anything that'll allow you to get beyond surface level.
If you're not fortunate enough to have this kind of diversity in your area, you can expose your child to another culture by visiting cultural event in a nearby city. Holiday times should have these opportunities available. Another option is to visit a religious center that has services in another language.