My son was the same i tried everything including making him sit on toilet 3 times a day but it was just a case of waiting he grew out of it himself as im sure your son will too GOOD LUCK!!!
A close friend had the same problem. She was called in to change his diapers every day-he had to wear them, no other choice. Maybe he's just not ready for school-usually an indicator is potty training. I'm sure they have a size big enough until he's ready. I hear a lot of boys have trouble with pooping on their own at this age. I'll know 1st hand in a couple years. I wish I had a better solution for you.