Bowel problems with my 4 yr old

My 4 year old has been potty trained for over a year now. About 3 weeks ago he stopped having bowel movements on the potty
He never indicated in any way that the bowel movements ever hurt him, he just now refuses to sit. We have him on Miralax now and
and he has bowel movements about 3 or 4 times every day. He will not go in his underwear, he changes into a pull-up and then
goes. We tried last night to make him sit on the potty by giving him an enema first then making him sit. He just cried that it hurt and
and kept crying for a pull up. Finally I had him stand up on the potty seat and then he released his bowels while he was standing.
Is it possible that there is something keeping it from coming out while he is sitting? We are trying to be patient but just do not
know what else to do. Thanks for your help