E.M.
Well, it could be a number of things. I'm guessing it is an age difference thing. Is the little girl in Kindergarten? Kids seem to become aware of their ages when they are in elementary school because they are around a lot of older kids. They idolize the older kids and don't want to be called the "baby" or "little" kid.
Now that they are focused on being big, it may be difficult to play with the younger kids. My daughter (6) played with a 10 yr old girl during the summer and often her 6 yr old brother didn't want to play (too hot). She became attached to her and disappointed when school started that this girl didn't want to play.
We explained to my daughter that we know she was buddies with a little girl across the street during the summer, but now that school is back in session kids are wanting to play with kids in their classes. We explained that the little girl can still like her but that she likes to play with kids her own age because they have a lot in common (friends, teachers, experiences, etc). We related it to the age difference between her and her sister (2 years) and how sometimes it is fun to play things with her and some times she just wants to do things with her friends who are in her classes. Eventually, when the newness of the situation wears off, she will most likely want to play again periodically.
Question is, does this girl you speak of have access to play with friends her own age (or older) in the evenings? If so, she may be comparing and choosing the older.
I would consider finding alternative activities for your son for a little while and try to give the little girl some space. Tell him that she may have some things that she wants to do on her own during the evenings and that he needs to be patient. Then in a week or so, have him go see her. Try again in a few days. Let her have a little space and she'll probably want to play again. That's my thought. She's probably just trying to wrestle with the big kid/little kid identity and doesn't realize how it makes your child feel. Most importantly, be careful not to make you son feel like he's not important or like he's not big enough to be important. Just offer that kids sometimes like different friends and friends their own age... something he can't change so he doesn't feel compelled to try to change it.
(Also, maybe you could quietly ask the mom if she thinks her daughter is just wanting some space or if she is wanting older interraction. You could help facilitate a better bond if you help your child respect the girl's wishes.)
Good luck.
Liz