OP, I'm so glad you picked up on your son's cues and were able to talk to him in a way he felt free to share. Great job not shaming Freddy or his friend. This is definitely something not to ignore - can you meet Alex's mom (away from the kids) and let her know what happened? You might also check with your pediatrician about how to follow up with Freddy at this point. Definitely keep an eye out for any unusual behavior or preoccupations. This article has some general info and a # you can call if you want to run this by a counselor:
http://bit.ly/1UnX75a You sound like a great mom - kudos again for being on top of this!