Regressed potty trained 3.9 yr old

My son was the easiest to potty train.  When he had to go, he would announce it to the world.  We would go to the bathroom and he would poop immediately and be done.  About 3 weeks ago, I think he had a painful bowel movement which scared and hurt him so much that he will not poop on the toilet anymore.  He has been holding it for 2-3 days.  He walks around holding himself, whining and wanting to be held for hours at a time.  I try to let him know that it will not hurt and that it hurts more to keep it in his tummy.  He seems to be getting worse and holding it  for longer periods of time.  Eventually, he can't hold it anymore and he poops in a pull up.  He says that it didn't hurt and that he will go on the toilet tomorrow.  The next day it starts all over again.  I have been loading him up with fiber and stool softner to make sure they don't get too hard and painful.  After reading many of these similar situations, I am feeling like I have a huge battle on my hands that could last years.  He was doing fine less than a month ago and I don't know what to do.  His doctor was extremely condescending when I brought him in explaining to me like I was 5 that constipation is not when a child holds their poop for 3 days. It takes at least a month of irregularity to consider it constipation.  Anyway, please help me!