This may be the beginning of the separation of the foreskin from the glans. This can occur slowly over time and does not have to be all at once. Urine at this point will likely begin coming into contact with the glans and aid in the separation process which can result in a stinging or burning pain.
Another potential is that he may be in his diaper to long and this may lead to irritation. Circumcised boys will sometimes develop blisters on their glans from their glans coming into contact with urine and then being held in that position with a diaper. Allowing your son to be without a diaper would be a way of determining if diapers are causing this irritation to your sons foreskin.
Insertion of a catheter does not require that the foreskin be retracted in any way and any physician that believes that it is necessary to retract the foreskin to insert a catheter is responsible for medical malpractice. This information is taken from the doctors opposing circumcision website.
Forced retraction is only rarely defensible. Inguinal hernia, undescended testicle, hydrocele, and varicocele can all be addressed without forcible retraction. The penis should be repositioned for any necessary intervention with sterile tape, not by inserted hemostat. Catheterization can be done by 'feel,' without retraction. Only in the instance of significant hypospadias or epispadias (congenital malposition of the urethral opening) might retraction be necessary and even then only if it is unavoidable collateral damage for which there should be specific follow-up care.
You can read more about this here: http://www.doctorsopposingcircumcision.org/info/info-forcedretraction.html
I hope that all works out well for you and your son. Please keep us updated if his condition gets worse or you notice anything unusual going on with his foreskin.