A.,
Yes, you definitely need to talk to the boy's Mother. Yes, the rock-throwing may have been an accident, and it is possible that she does not know how her boy behaves. I would be concerned about the underlying characteristics, though. This boy already seems manipulative, seems to lack empathy for your son, and appears to encourage lying. He might be a nice boy with a need to be liked, and merely accident-prone, or he may be on the track of developing into an uncaring psychopath. OK, the chances are not high for that, but I would check his behavior to small animals and little kids, just to assess empathy. I would voice your concerns to his parents, not in an accusing fashion, but just to determine if he really is your son's friend or if he is using your son as a convenient target under the guise of friendship. At the very least, he could have walked your son home to make sure the bump on his head wasn't serious, and explained to you that it was an accident. At 10, he should know that rules apply to him, and caring is important.
I hope you get this ironed out, and that the Mom understands the dynamic. Best wishes,
K.