Oh goodness- I cannot believe they are telling you there is nothing they can do. Who told you this- his general ped? His cf dr? They can reduce it for you at the hospital. That is terrible that your son is hurting this way. <img src="i/expressions/face-icon-small-sad.gif" border="0"> My dd has had rectal prolapse- never to this degree though- it's always gone back in on it's own. She saw a surgeon for it twice and we determined that she did not need the surgery that Jada mentioned above. They told me that if it ever did not go back in on it's own, to try to push it back in, if I couldn't, to immediately get her to the hospital and they COULD reduce it and they would reconsider the need for the corrective surgery.
I would take him to the hospital (a different hospital if it was the hospital emergency room telling you they couldn't help you) and get an 2nd opinion (tell them it's an emergency) with a reputable pediatric surgeon for this. I hate hearing how much pain he is in!
eta> they can do the procedure Jada mentioned above, there is also a procedure they can do involving the outlet muscles of the rectum/anus. A thorough exam by a ped surgeon who knows what they are doing can help determine the source of the problem with your son and which procedure would be most helpful.