Actually, I would say that a call to the diocese is definitely in order. Unless the details you know were told to you by your son, the fact that you have any knowledge regarding what went on during his confession is extremely problematic. I’m ignoring for the moment whether refusing absolution is appropriate or a good idea, as I am more concerned about the seal of the confessional. Perhaps it might be allowable for a call to be made to ALL the parents informing them that the priest found some of the class to be insufficiently prepared to receive the sacrament, for the priest to clarify what he considers to be well prepared, and that he only provides absolution to those he considers properly prepared. However, whether any particular individual received absolution absolutely should not be disclosed, nor why any particular individual was denied absolution. Absolutely nothing that went on inside the confessional may be revealed in any form or fashion.
The ONLY person who could legitimately reveal any of the details, whether that he personally did not receive absolution, that he had trouble reciting his prayers during confession, or that the priest believed he had not thought hard enough about his sins is your son (or someone who your son has previously revealed this to outside the confessional). I disagree with some earlier posters regarding whether this may be breaking the seal of the confessional. If the statement were that your son was denied admittance to the sacrament, that would be one thing. A priest might legitimately ask him to recite some prayers to demonstrate understanding prior to even beginning the sacrament, ideally prior to entering a confessional (to be clear that the sacrament has not begun). However, any questions concerning the nature of his sins absolutely cannot occur outside the confessional (anything more specific than have you thought about your sins to prepare to receive the sacrament). Therefore, unless your son told the priest outside the confessional that he had not thought much about his sins, there is no way the priest should be able to say your son has not thought hard enough about his sins.
Of course, I am also concerned about a priest denying absolution; the requirement of imperfect contrition is not all that demanding. While I understand that absolution is refused to a person who is invalidly married until that person professes an intention to refrain from further sin, there don’t seem to be the same sort of opportunities for a small child to be obstinate in their sin. Certainly, when they confess to lying, both they and the priest may be aware that they will likely lie again in the future, but that is not a barrier to absolution as long as they are currently sorry. If I were a priest, I expect I would likely only deny absolution to a small child in cases where they either tell me in confession that they are not sorry for it, that it was worth it, that they would do it over again if given the choice, or that they intend to do it in the future (and not just that they are afraid they will fall into the same sin again).
In short, there are potential ways that this could have not violated the seal of the confessional, but I would expect that even the appearance that the seal of the confessional being violated, is sufficiently serious that the bishop would want to know.
