Poo toilet training problems help

My son jake has been toilet trained since 2 yrs old now he only will poo into a nappy. His wee's are fine he has never had an accident with wee's so we are total toilet trained in that department but i just cant understand this whole poo in the nappy thing im always positive with him. i've tried not putting a nappy on him and just ignoring it and putting him on the toilet but he just holds on until he cant hold on any longer and poos himself. I really need some advice if any other mums are going through this im total stumped for solutions. Its like his ashamed or sometjhing but we have never done anything to make him feel ashamed.

HELP

I just wanted to say I am having the same prob with my 2 1/2 yr old. It seems to be very common. I started a poo potty chart and he gets a sticker if he does one. (he's only had one after doing one tiny poo). When he gets 5 stickers he'll get a prize. So other than that, I try to give him lots of praise if he tries. I try not to make a huge deal of it when he does do it in the nappy. I have read some other messages, I am going to try putting a nappy in the toilet and also buying a special step or seat especially for sitting on the toilet and trying to do poos. The nurse advised me that they actually enjoy doing it in their nappy in bed because they are relaxed and no-one is around.

We have a toilet seat for the toilet for him he know gets so cranky if we ask himto sit on the toilet we even leave the toilet so he can be alone and he just plays with the toilet paper im so worried if i dont get this under control he will be one of those school kids that poo their pants it terrfys me. The thing i have noticed with him is when he poos he stands up when pooing not crouched down. I tried putting a nappy on him and placing him on the toilet that also didnt seem to work. I feel like a failure that i havent got him toilet trained in the poo department, alot of my friends boys are completely toilet trained no hassels, I feel as though im doing something wrong. Thanks for your reply chicky