My 8 year old was 3 before she was potty trained to poop on the potty. Same situation, she was starting pre-school and was getting down the pee in the potty process, but kept pooping on the floor, the porch, in her panties, etc. I was trying so hard to stay positive and not get stressed, but I had arranged to start a part-time job during her pre-school hours and she had to be potty trained to be in school. I read the Princess and the Potty, Miss Piggy's No More Diapers, got fancy panties, panties with characters she loved, gave out M&Ms, etc. It just wouldn't happen.
Finally, she started pre-school, they took bathroom breaks, and she had one accident the whole year, and it was a pee-pee accident.
My son trained "socially" too. I worked with him for 6 months before he started part-time care at a friend's home who had a 4 year old son, and PRESTO, he was potty trained. I have talked to other Moms who had the same experience. It seems like something clicks in their minds when they spend several hours a day away from Mommy with other kids who are already trained. They are on such a routine at school, also, and it is healthy for them to have a routine they can count on every day, and their bodies just depend on the rhythm.
I say, keep smiling, keep everything happy happy happy, and try to not let him know how important it is to you that he get trained.
Best wishes and Good Luck!!
R.