Check for constipation. My daughter had severe constipation for a year and was soiling her pants. The new school or bathroom perhaps may have caused him to hold it. Mine would hold and then her body would just push it out and she had no control.
Ask the school if the children have easy access and are allowed to go to the Bathroom when they need to, does he do this at school aswell?as it strated when he has gone to Pre K I would think the problem lies there so make enquiries there if he is holding it till he gets home then there is a problem with him having access at school.