My older sister was not an only child, obviously, but definitely the signing up for something and not wanting to go was a type of thing she used to do (she would really freeze up and/or make a big fuss), and also reacting very, very strongly to things said that weren't necessarily that warm and friendly but things kids say (she could actually be pretty mean herself). My mother used me as sort of a social/emotional "companion" to her so I had to get signed up for anything she did, but it didn't always cure her from freezing up, but my mother would pretty much just push her in the door and take off. Did the choir director help you get him in the door and make him feel comfortable? Did you talk to him/her afterward, will your son try again?
I would say counseling would help if it's a good fit - did you ask them how they would approach him (i.e. like a victim or someone who needs to learn more about himself and some new tools his parents don't know about?)
I think the kid saying something about what happened 2 years ago is an indication that there might be some social dynamics he's having to deal with at school. Maybe it's not super bad but sort of "there". Why can't that boy let the incident from that long ago "go"? Everyone makes mistakes.
Have you read many books or articles about highly sensitive children there might be tips there.
