Thank you all for your opinions.
Actually, I was somewhat comforted that other children and parents noticed this child, and sort of felt that it was not my place to get involved, as I didn't know them at all. However, I did have a name to go on (thanks to my son!) so after I got home, I did some sleuthing in the school directory and determined who the child was - and that they are in Sp.Ed. for emotional support. (STP, are you a psychic, or do you just play one on TV?)
I think I probably did less than I probably could (point my son back in the appropriate direction and prod him to re-initiate a conversation, for example), but if I had done more, I might be thought of as a stalker. Plus, he wanted to go, and he came first. So, I'll just keep a half an eye on things.