3 yr old boy stopping his BMs

My 3 yr. old boy has been holding in his BMs for a week or so.  We have been potty training and the urinating part has been a great success. But, when he sits on the toilet to try and go poopy, he tells me he can't.  I don't push the issue, figuring that he will get used to the idea soon enough.  But about a week ago, he began to squeeze his "cheeks", his body shaking because of his effort to hold in his BMs. It is causing him discomfort and even fatique at times. I have given him a laxative to help the buildup. Does anyone have experience with this?
Thanks

[Kevin Kennedy, Ph.D.]

This is a common problem, and usually originates because children withhold stools, then become constipated, resulting in impacted bowels. The withholding of stools is brought on by fear of pain while moving the bowels. It's best to have him evaluated medically. If his bowels are impacted or he is very constipated, a regimen of laxative and stool softener will be necessary for some time to clean out his bowels and establish regular stooling. Don't try to manage this on your own, though, without arranging an evaluation with his pediatrician.