Help, My 5yr old poo's His Pants?

I have a 5yr old boy, which I have enrolled into school for 2008. He WAS fully toilet trained but now poo's his pants. I have tried everything!!! Ignoring him, bribery, banning him from treats, Charts, etc. I have taken him to the doctor and they just tell me that he will grow out of it....that is not much help!! I have a 2yr old Daughter that has pretty much trained her self, She has had 1 dirty nappy in 3mths. I am very concerned that he will be teased at school, I do not want this to happen because it will stick with him for years to come, He already gets called "Stinky" at preschool. I am in urgent need of somes advice - preferably from someone that has been in my shoes. I am at my whits end as I am expecting my Third child in a matter of months and I dont want to be chasing 3 children around changing nappies all the time... Someone PLEASE HELP ME!!!!!!! It would be greatly appreciated.