Have you talked to his mom to see what might be up? That seems like a drastic change.
Iwonder if the other boy is resentful of your son because he is the punishment if he does something wrong? I know it isn't your sons fault but maybe the other boy feels that if he doesn't play with your son, then his mom won't ground him or he doesn't have to face losing your son all the time because of punishment? Does that make sense at all? I would definitly go talk to the mom and see what's up.....
Good luck and wish I had more advice
I'm wondering if Laura might not have hit on what is making this kid want to avoid your son. I think the mom caused some problems when she used time with your son as a reward and taking it away as a punishment for her son's behaviors.
Have things changed since you posted?
Actually funny enough - just saw your post and his friend called on the weekend but we weren't home - I have been trying to keep him busy with other friends.