Help! My almost 4 year old wont poop on the potty

My oldest son has always had poop issues. we had a bout of him being consitpated, had to use suppositories (trauma!) etc. We've never had a problem peeing on the potty, but he won't poop on the potty for anything! Our current reward system is: he has to poop on the potty 5 times (using a sticker chart) and then we will get a Transformer toy. He understands the deal, and has even woken up at night telling me about it. During the day, however, I do not put him in underwear or a pull-up. He will poop in his underwear etc. so i tell him that if he needs to poop, we'll go sit on the potty. we sit on the potty, he pees, then tells me that he doesn't want to poop. a few minutes later, he'll ask for a diaper or pull up. (I have another son who is 1.5 and not potty trained)
It's been three days of no poop, do i keep going with my plan? or do i need to revise? Help please!

[Sweetipi]

I just had my 4 year old son potty trained... Like yours, he started peeing in the toilet and asked diaper when he wants to poop... But what we did is everytime he wants to go and ask us diaper, we just told him no diaper and it's either he will poop without diaper or go sit on the toilet... It will take a while but so far he never poop on the floor but instead he ran to the bathroom and call us when he's done... Just be more patient and talk to him more:)