Pooping in the potty

[Whiteangel]

My son is 4 years old and he poops in his underwear all the time. He disappears into another room and poops or he picks at his bum and when i ask him if he has to poop he tells me he already has.  How can i get him to poop in the potty?  I also have a 18 month old daughter and the doctor said he could be pooping in his pants because his sister does it, but i am so frustrated and just want him to poop on the potty cause i am sick of cleaning his underwear. What can i do to get him pooping on the potty. I have tried the i will buy you a small toy trick and the i will give you a treat trick and they just do not seem to work. I am really frustrated and worried he will never poop and the potty. Someone please help me.

[cambugsmom]

My son will be 4 in June, I have tried everything to get him to poop in the potty...Rewarding him is the worst thing i've done, because if he doesn't get what is up for grabs he just finds a new interest. I've tried making him sit on the potty or just refusing to put a diaper in him, but he will hold it for a whole day and I can't let him suffer like that. He has no problem peeing in the potty at home, he goes completely on his own standing and everything. He always asks for a diaper when he has to poop, after he does, he just asks to be cleaned. I don't understand how he can know he has to go but just can't go to the potty to do it. Maybe if someone could just explain to me why that is I can do something or atleast be more patient.
I've put off potty training my 2 1/2 year old daughter because i was so comsumed by getting him trained, therefore, I make him help her...Take her to the bathroom, turn on the light, put the potty chair on for her, and then leave the room. Then I praise her for going, which she loves. So far he still has no interest.
He went out for the day with his Daddy and I made him go without a diaper. They were gone for around 6 hours. When they came back I was informed he hadn't gone at all, not even pee. He immediately wanted a diaper on, I put a diaper on and he went.
I don't know what else to do, I read everything everyone else had to say and I cried lol because every answer I have read is one I have already tried...