Hi Kendra,
Congratulations on meeting your new friend and her wonderful family. I am a mother of three, ages 11, 6 and 2. My six year old son has Down Syndrome.
All I can say is don't make a big deal about your friend's child being "different". IF you son asks questions then answer them truthfully. There is nothing to be ashamed of and kids are not as shy as adults to ask.
The only thing "different" about your child and a child with Down Syndrome is that the child with DS has an extra chromosone. It is the same as if your son has his daddy's red hair and your perfect lips. Each child is different in his or her own way, that is what makes us unique.
Ask the mother if she is comfortable talking to your son about what makes a child with DS unique. Kids are fasinated by the fact that my son has a line across his hand (a sign of DS) and their "lifeline" curves.
Being upfront and honest with your son will make him more comfortable to be around the other child and because he knows what the "difference" is, he may be more likely to stand up for others in the future.
Let me know if there are any specific questions I can answer for you.
Again, congratulations on your new friend.