OK... Talk with the Preschool's Director -- that's just unacceptable and negligent, to send a child home with poop in his pants.
Work with your Preschool, and at home, to create a "standard" procedure to help the situation? For example, when he start's whining, stop the fun until he's tried 5-10 minutes to go poop. Maybe a special book or toy for only when he's pottying at home or in school?
Body functions are very important -- maybe a fun picture book to teach him how we all process food? Kids are really smart when you're straight-up about negative AND positive -- have you explained to him that you are medicating him?
Maybe a portable DVD in the potty to "reward the wait"? My son will do almost anything to watch TV:)
Our kids know we are a little "lazy" too -- my son will work me like bad playdoh until I put my "big girl" pants on and just do what he needs me to do. (Hahaha... do do....) It's hard on me but I need to teach him the "limits of living" -- and have a big breather when the tantrum's done:)