In addition to the great advice that you've already received, I would also try to find out if anything happened to him in the bathroom at school prior to 4 days ago. If he had no problem with this before and now is refusing to use the bathroom at school, I would consider that something might be scaring him from going in there. It could be anything from another child teasing him to someone trying to hurt him. Very calmly ask him if something has been bothering him and if something happened to him at school that he wants to talk about. Pay close attention to how he answers...or refuses to answer, and to his body language. You can also contact his teacher and ask her if s/he has noticed any kids at school bullying him in any way.