4 years old refuses to potty

My 4 years old son refuses to have a bowel movement on the toilet. He tells us every time that he is scared. He will pee in the toilet and holds the 2nd at school and when he gets home he asks me to put the pull up on so he can potty. He is also a very picky eater and constipated a lot which I think that could be a problem. What can we do to help our son to use the toilet. We are worried that he will not be trained before he start the kindergarten.
Please help

[Kevin Kennedy, Ph.D.]

First things first. Solve the constipation issue and then it will be more comfortable for him when he moves his bowels. Confer with his doctor about the plan re: constipation. Your son may have to go on a regimen of laxative and stool softeners for a while, along with making modifications in his diet. Be sure to do this with the guidance of his pediatrician.