Potty Training

My almost 5 year old pees on the potty, will sit on the potty and read a book, but will not eliminate on the potty.  He is holding it in so much that he is getting stomach cramps.  He constantly is spotting his pull ups because, whilke he is holding it in, a little poop seems to creep out during the day.  His preschool is becoming very frustrated constantly having to change him during the day.  What can I do to help him get over his fears to actually go on the potty ???