I'd recommend you get the book "Mommy I have to go to the Potty".
For boys, it is harder for them to get. I feel you are expecting too much and you are not hearing him...
In the book, some kids fall into the thought process of "everything will fall out, if I sit on the potty and poop!".
As for the other kids, I'm sure they too, were using the potty. If not, then yes, it was more of a goal...but not a realistic goal. He is getting close and you still have to remain PATIENT!
You also have to remember that he is different from your daughter (and boys mostly train closer to four years of age). Girls WANT to be clean~ boys could care less in most cases.
I recommend you be patient, calm and show support. Sit down and TALK with him. My son is five and he and I have had so many conversations about things that could be bothering him or not.. the point is, I GET Down to his level of thinking. Parents tend to forget this important factor. Communication goes both ways, and even at his age, he can talk to you.
Introduce POSITIVES, but try to avoid the negatives. Taking away right now is not going to help him go potty. This is his deal, not yours. Help him with it by going thru it with him (on his thought level.).
And no..I do not agree with either poster below as that can be traumatizing to your child and can possibly introduce a set-back later when he is stressed at Preschool. My son (and my nephew) both reverted due to stressful events after being potty-trained. (We had a change over in teachers and a boy was locking kids in the bathroom.)
You have to take a positive, loving and nurturing approach. Even then if it occurs.