Getting my son to poop in the toilet

[Trinty425]

With my first son, we introduced the potty at 2 and started training at 2 1/2.  He went poop at almost the same time everyday.  So we just put him on the toilet and I entertained him with books and toys for almost 3 hours.  Then he finally pooped and we threw a party.  Once he realized what he was supposed to do he had no problem going.  And was fully potty trained day and night pee and poop by age 3.  

My second son however is quite different.  He is now 3 1/2.  We introduced the potty at 2...no interest...at 2 1/2 he started showing he was ready.  He quickly picked up going pee sitting, and then standing.  He rarely has a pee accident, even when we are out of the house.  About a year ago we did get him to go poop in the potty by accident.  And we threw a party and he loved it.  But...he hasn't gone poop in the toilet since.  We have bribed him with toys, parties, candy, everything that interests him.  We have had many talks.  He isn't scared of the toilet or of pooping.  He doesn't think it's yucky or anything.  He does not wear diapers at all, and only wears pull ups when we are going to be out of the house for awhile (in case he poops..he always goes pee in the potty even with a pull up on).  He knows that he is supposed to go poop in the potty.  He has watched me and his older brother go poop.  He understands that everyone goes poop.  Even with him wanting very badly to go to school, and me telling him he can't go until he goes poop in the potty doesn't phase him.  I have been upbeat with him, disapproval, tried bribing and rewarding anything and everything.  Tried taking toys, and such away.  Like if he was coloring when he pooped, then he can't color anymore.  He doesn't even have a behavior showing that he might be pooping.  (like asking for a pull up, hiding by himself, anything).  It doesn't seem to bother him that he poops in his underwear.  
He doesn't get constipated.  He has no physical issues.  I have asked him many times if he is scared or anything about poop and the potty. He says he really wants to go poop in the potty like a big boy.  But he just doesn't.  He goes pee on his own.  Can take his clothes off easily and put them back on by himself.  I tried to have him go naked...but he has hates being naked.  He has never liked being naked.  I have done a good bit of looking around online, and I have tried every bit of advice and tips that I have seen (except the laxative...I will NOT do that to my child).  I am clueless as to what to do.  I don't mind waiting for him to go on his own...but I'm afraid that he will still be pooping in his underwear when it is time to start preschool / kindergarten.