Greetings CaraT:
Sometimes small children will hold their BM due to a painful bowel movement. One painful bowel movement can trigger a cycle of sorts. The first painful bowel movement can lead to behavior of trying to "hold it" in order to avoid the
pain. This leads to larger stool which leads to another painful BM. Watch his behavior and if he seems to be attempting to avoid going potty this could be the problem. As always, be sure to keep your son's physician aware of any new or worsening symptoms.