There definitely are children who never develop comfort with using the restrooms at school. They will go to great lengths to wait until they are at home to use the bathroom. In and of itself, this needn't be a major concern, though of course it would be preferable if your son were more at ease with using the bathroom at school.
The benefit of having him see his doctor is to rule out constipation. This condition is more likely to occur if children are avoiding using a bathroom, for whatever reason.
Yes take him to the doctor. Here is a link on encroprisis. http://www.med.virginia.edu/medicine/clinical/pediatrics/CMC/tutorials/constipation/encopre.htm
He could be using not liking the bathrooms at school as an excuse. There could be a underling cause besides cronic constipation. He needs to see a pedatric gastologist.
My son is now 6 and has the above. At one point in time his poops were so large they filled the toliet. We had to flush before we wiped.
You might want to get your son sitting on the toilet after eating for 10-15mins to see if this will help him go. Good luck.